I play it occasionally (mostly just when new DLC comes out now) but I'll be honest, I don't play co-op. I'm not nearly competitive enough, and I like cheesing the game with all my mods.
On that end, I can recommend this website
for mods, this one
for XV patcher (necessary for making mods work right), and this one
for the character installer (which also allows you to reorganize the character roster).
Personally, I have some go-to mods (Tarble, King Vegeta, DBS Trunks hair color fix, Bulma, a few DBZA OCs, etc) but the sky's pretty much the limit. If you need help setting up, just let me know.