Welcome back!
The game has changed a LOT since you’ve left and the vast majority of those changes were for the best. If you choose to stick around a while you may find that the end game of 5*s is an exercise in frustration and slow progression but there’s hope for further changes in the future so please don’t let that discourage you.
As for tips?
Happy to help! Keep in mind the suggestions I’m providing here are for LONG term success. If you just plan to stick around for a few weeks or play fairly casually than by no means feel pressured to do this.
1) Don’t spend CP or Legendary Tokens. Period.
Optimal play and building for 5* tier is all about hoarding right now. Your goal is basically to save 1000’s of CP and hundreds of tokens (ideally for 300+ pulls but the more the better) and then blow them all on a good pack of Latest Legends. In doing this, you should (luck depending) be able to champion three 5*'s which will instantly elevate you to the endgame.
So what do you do while waiting for that long term goal?
2) Champion EVERYTHING
Champion your 2*'s, 3*'s and 4*'s. They all provide excellent rewards that speed up your progression and flow of rewards in the long term. Try to focus on characters that will provide immediate value at first but once you have a somewhat healthy roster than just champion whichever character the game is giving you the most covers for.
3) Cycle your 2 champions*
What does that mean? Once your 2* champion has reached maximum champion level 144 they can no longer gain champion benefits. We can’t have that! Optimal long term play would have either build a 2nd version of every 2* OR simply sell off the existing 2* and rebuild them again. This may sound nuts but in the end you basically net lose around 8000 ISO but gain a ton of champion reward benefits that easily make up for it. Be sure you champion the 3*s whose covers you get from these 2’s so those rewards do not go to waste.
4) Use the forums/your alliance to help out with who to level.
When building your initial army you’ll want to take advantage of your resources to get a feel for who’ll help out the most. The current meta is actually pretty healthy thanks to D3’s commitment to balance characters and a weekly buffing system that ensures different characters get the spotlight. It basically means that having a diverse roster will help you in PvP and PvE. And so building up a good foundation of 10 to 15 3’s and 4’s should take priority so you don’t find yourself in too many events where you have no good boosted characters to use.
5) Play for progression.
One of the better changes in the last two years has been the improved progression rewards. This is especially true in pve. Don’t feel like you need to grind out for top 5 every event unless that’s your thing. You can clear each node 4 times and easily get all progression rewards plus a bunch of bonus covers for what will likely be top 200-300 finish. Doing this will cut down on the grind and potential burnout while maximizing rewards.
The same is true for PvP but you’ll hit a natural wall here based on your roster so things are more outside of your control. Don’t be discouraged if PvP success doesn’t come quickly, do what you can and milk the PvP, deadpool daily, and lightning round rewards while you can.
6) Lightning Rounds are you ISO goldmine.
You may remember this from before but in case you don’t…join LR’s literally within 15 seconds of them opening up. Doing so should ensure a bunch of seed teams for easy ISO and progression ISO. Do this as many times as your schedule allows each week and it will make a substantial difference in your ISO flow.
That’s all I’ve got for you. It should keep you busy for at least a year…by then you might be ready to jump into 5* tier play as well 
