@OP: Yeah, you want to roster the non-Bagman 2*s . . . IF you are playing PvP, PvE and/or Lightning Rounds regularly and generating tokens. Which contextually from your post you are, or else you wouldn’t have excess covers, because where else would you get the covers?
But to be clear - IF you are generating tokens regularly, then you roster the non-Bagman 2s because they’re generating CP, HP, and tokens. You don’t sell 2s off to roster another indifferent 3*, because though any 3* you roster will get you the next tier of rewards, those rewards are far slower to earn. This is even more the case for 4* rewards.
IF you are NOT generating tokens regularly (say you are only playing Deadpool Dailies) and say you are REALLY doing NOTHING else then you dump your 2s because you don’t need them as Essential characters, you’re not generating tokens, and your primary HP income is through having the 3 Essential character for Deadpool Daily Quest.
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Other note: Roster 1* Juggernaut. For Deadpool Daily Quest. If you happen to have XYZ characters, and you’re at that point, you’ll need to clear the 1* node for rewards. Roster no other 1s though. Others might say "roster 1 Black Widow with five blue covers" - yeah that’s great if you have roster space, but you don’t, and you won’t for quite some time, so don’t worry about it.
If your alliance demands you roster 1* Black Widoww with five blue covers - just find another alliance. You can still get in a top 100 PvE alliance, and that’s about where you can be.
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As to your goal should be covering the 3s - no. That’s nice for veteran players that did things that way years back when there were fewer characters, or whatever, but these days - no. These days, you roster 1 Juggernaut, the non-Bagman 2s (provided what I wrote at top), a selection of 3s, and select 4s. Mind the word “select”. You’re not just going after whatever indifferent 3 or 4*, you’re going for the ones that will increase your ability to perform in PvE, PvP, and alliance events, and that will decrease the real time spent to earn rewards.
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And I’m gonna say yeah, you do want to dump some 3s for those preferential 4s. More on that in a bit.
OK the characters you should concentrate on - IMO you shouldn’t screw with a lot of character ranking lists that are out there, because they’re mostly geared from what I’ve seen towards players in the late 3*-4* transition and PvP. But that is not you. You’re maybe going for PvP but probably you are going for both PvP and PvE and you don’t have level 250+ 3*s so boosts and priorities will be different.
Priority 3*s:
Iron Man, Doctor Strange, Hawkeye. These are your bread and butter; Iron Man accelerates your AP, Doctor Strange punishes enemies that fire active powers (typically non-character enemies that fire powers that create countdowns; Strange hits them with immediate damage and heals your team; meanwhile the countdowns need time to count down before they cause problems). Hawkeye messes with enemies that create Strike, Protect, and Attack; Hawkeye generates AP off those matches and has true regenerate so . . . yeah.
Captain America, Kamala Khan, Hood, Thanos. These are next tier. Captain America can be charged up quickly with Iron Man; in conjunction with Kamala Khan each time he fires temporary hit points are generated. Plus Kamala Khan covers purple and green and has an okay-ish (not great really) team damage power. Hood steals AP, and though Hood is super fragile he’s important in some matchups (though rare so he shouldn’t be highly prioritized). Thanos passive black deals a load of damage, which saves you time. Thanos’ damaging your own team may seem a drawback but it won’t be so much once you use disposable characters and use Thanos properly.
Deadpool, Scarlet Witch, Magneto, Blade, Iron Fist. Deadpool so you can earn points for Deadpool Whales, Scarlet Witch for passive board shakeup, precision green control (usually won’t end up using it but it’s nice to have) and purple power. The randomness of Scarlet Witch’s stun makes it seem not so great, but it’s a 5 turn stun, which is pretty great in practice. Magneto is unique in that he has multiple board shuffle (not destroy) powers which can come in uniquely handy, plus he has a blue bomb. Blade generates Strike tiles so ends up being useful in a lot of combinations with other characters. Iron Fist can cut your easy PvE node times with a powerful persistent Attack tile, and in longer matchups his purple can be abused for a load of cheap damage.
Black Widow, Daken. Black Widow has multiple precision-board-manipulation powers. The crazy cost of her powers might make her seem lame, but in practice she’s often a go-to against alliance bosses. Daken is mostly lousy, but he regenerates and has a two-step damage power that is useful with Thanos.
Other stuff okay might seem nice. But think on this - usually other characters are in there for just one big power, and you can use alliance Team-Up powers for that. Thinking about using Black Panther for his black team damage? (Which granted is pretty nice). Well instead of using a 3*, you can use a 4* like Jean Grey that additionally removes Special tiles or generates Special tiles. Or thinking about using 3* Daredevil to get rid of enemy Strike, Protect, and Attack tiles? Daredevil’s purple is expensive for what it does, but 4* Spiderwoman’s purple is cheaper. Throw in 4* Riri for teamup powers, and you’re okay for most situations.
On the other hand, though you can use some of the above listed priority 3*s for team-up powers, usually you want them around so they can keep using their powers (Captain America and Magneto and Iron Fist and Black Widow and Daken) or benefit from passives (just about everyone).
Priority 4*s:
Rocket and Groot, Medusa, Gamora, Carnage. Then Captain Marvel, Agent Coulson, Vulture, Peggy Carter, Kraven, more I won’t get into here.
Reason 4* Rocket and Groot is nice is those Strike tiles. Which may seem pretty lame because you’re thinking “but I’ll match them away”. But if you combo with 3* Strange at PvE SCL 7, non-character goons don’t make matches and even if there is a matcher in there somewhere, you’re doing huge damage so usually stuff just dies.
Medusa can gain health off enemy special matches, which makes her really strong against enemy Sentry bots - and though it may seem that’s a pretty specialist thing, she also can be handy in PvP, PvE, whatever. Whenever enemy Specials are around, Medusa’s good, and that can happen a good bit.
Gamora is nice because she makes 4* Rocket and Groot’s Strike tiles stronger. Again, you may be thinking “but they’ll just be matched away”. In the right matchup, stuff just dies before that happens. So she’s good to have on the team. Plus she has a nasty stun and a insta-kill power.
Carnage generates specials for Medusa.
Captain Marvel often lets you get AP ,and you can customize your team with equipment sometimes so her AP generation power also deals damage. Coulson combos with Captain Marvel. Vulture is . . . well just look at him. Peggy makes enemy powers more expensive, so in some situations is really strong. Kraven cuts enemy special tiles down in effectiveness which can be very good.
There’s others I won’t get into here. But basically the priorities are stuff that cuts real time spent in matches (hence why Strike tiles are good) or that increase viability in tougher matches (like Medusa).
For the listed 4s - um I can’t really say. 4 Wolverine is given out as a free character for daily rewards sometimes and though he’s not as good as he once was by far he’s not the worst I’d say. Thor’s kinda lackluster imo, not bad, but not great. Star Lord can be pretty good. Ant Man is situational but good in the right situation. Domino I think requires a more specific team to really be of use. Elektra same. Iron Man okay. But basically they’re mostly “okay” rather than “higher priority”.
As to what to dump for what - I’d say dump the single-cover 4s that aren’t that great and probably won’t be, though you can look for another opinion. As I see it, you’re not going to be putting more covers on those 4s any time soon, and you are really going to need roster space. I’m not saying dump them RIGHT NOW, keep them on until the 14-day timer on some of your other characters is going to expire, then sell one of them and roster the new character - in the meantime, maybe you’ll draw that particular 4* for an event or Deadpool Daily Quest thing or something.
In the long term though, your roster slots are going to be hard up and you will need them - because even if you keep a few of those 4s you have (not all of which I’d dump in your position), you’d probably be best off getting a few of the higher priority 4s I mentioned. (I haven’t even mentioned nearly all of the better 4*s, just some that are maybe a bit higher priority imo.)
As to 4* covers being harder to find - sure. But 2* and 3* characters gain rewards faster that you can leverage into gains. Again “select” characters is what you want. It doesn’t matter if 4* Sandman is rarer than 3* She-Hulk. 4* Sandman is generally not that great, so you won’t be keeping 4* Sandman anyways, you’ll be cycling him out to get another 4*. So you are going to definitely get rid of some 4*s, might as well make it the ones that don’t have too much general play value.
I mean sure yeah you could keep spending money and buy roster slots. But I read in the OP that’s not so much an option. So I’d say again - research what to keep and what to sell off, and remember you WON’T NECESSARILY WANT TO ROSTER ALL 3s BEFORE YOU START WORKING ON 4s.
Like yeah you probably want to gold-star (you know what that is right?) 3* Doctor Strange then Iron Man then Hawkeye (if you do PvE a lot, otherwise Iron Man first). And gold-star 4* Rocket and Groot. But it’s not like you want to wait to get 4* Rocket and Groot until you finish rostering all your 3s. Go for it as early as you can; play those PvE SCL 7s to get select 4 covers when they come up.