All Roster Spots being free would make a lot of things that are cool about MPQ non-existent.
A couple of things I would suggest; giving away a Roster Slot once a month in Daily Rewards (or just make it something that has a chance to drop in random prizes), Roster Spot tokens (as mentioned earlier in this thread) that can be given away in different events periodically, bundles of Roster Slots like at the start of your journey (this would encourage impatient/desperate people to buy HP, but it also feels satisfying to see those three free slots), and of course reducing the actual cost of Roster Slots.
If it were up to me to manage the way the price increase worked for Roster Slots, it would look something like this (note: incline in price is figurative, not solid numbers or percentiles);
#1-#5; Free.
#6; 50 Hero Points.
#7-#10; 100 Hero Point bundle.
#11; 75 Hero Points.
#12-#14; 100 Hero Point bundle.
#14; 75 Hero Points.
#15-#17; 200 Hero Point bundle.
#18; 100 Hero Points.
#19-#21; 250 Hero Point bundle.
#22+; Here is where I feel like they should jump in price a bit. It’s hard to defend the need for more than 20 Roster Slots to transition. If you want to be a collector, you should have to pay a bit of a premium. Like most people, though, I think there should be a cap. I feel like 500 Hero Points is acceptable, but 300 Hero Points is much more fair, since you can get that usually between a PvE and a PvP. Bundles of three would be something like 1250 Hero Points is they cost 500 a piece or 750 Hero Points if they cost 300 Hero Points, like I proposed.
Or something like this;
#1-#5; Free.
#6; 25 Hero Points.
#7; 25 Hero Points.
#8-#10; 75 Hero Point bundle.
#11; 50 Hero Points.
#12; 50 Hero Points.
#13-#15; 100 Hero Points.
#16; 75 Hero Points.
#17; 75 Hero Points.
#18-#20; 175 Hero Points.
#21+; Like I said above, this is where I’d say scaling would start kicking in. 125 Hero Points and 150 Hero Points for #21 and #22, respectively, #23-#25 would be something like 350 Hero Points, #26 would be something like 200 Hero Points with #27 being 250 Hero Points and #27-#30 would be something like 500 Hero Points, from here singles would be 300 Hero Points and bundles of three would be 750 Hero Points.
Both models are forgiving to new player and players trying to get through their transition. Both models offer fair and attainable collectors status. Both models encourage people to spend Hero Points on slots in bulk. Even at the end of the second model’s incline 5 slots would cost 1350 Hero Points. I feel like that’s chunky enough to persuade enough people to drop 20$ here and there to get those slots for the new few events, but it’s also low enough that a player that players semi-frequently can attain them without spending any money.