Cross Levelling

There's a strong temptation on superhero games to start helping other characters out with new gear and talent.

This is one of those issues that treads out of 'what's fun' and into 'what makes for a good story'. Yes, it would be awesome to give Cap's Shield and Thor's Hammer to Wonder Woman while she's wearing Vibranium armor and wielding the Infinity Gauntlet full of Lantern Rings.

This is really, really bad writing.

As much as I loved Infinity Wars, look at Parker getting the Iron Spider outfit. In two movies he went from 'sweat pants and goggles' to 'classic Spider' to 'Ultimates Marvel Alliance 2 Sweet Unlocked Special Armor With Max Stats'.

Try not to look at other characters as means for power-levelling, rapid advancement, or sweet rides and cherry gear. Look at other characters as /characters/, first and foremost. There are often good and proper reasons not to just hand out sweet swag-- Tony Stark might not trust someone with his Iron Man armor, or Diana might feel that training mortals too much makes them too reckless. Batman might not give his proteges the BatTech because he wants them to learn not to be dependent on it. Find a balance that serves the needs of the narrative, rather than trying to 'game' the game and rapidly kit your character out with all the cool stuff.