It's not going to be like Smash Bros. It's going be like OW regarding characters and unique abilities but it will play more like a Brawler like Anarchy Reigns. Characters will be able to do combos and combo abilities with other heroes on the team.
The only game mode we've seen so far is an arena type mode where you fight 4v4 in a huge arena with stage hazards being present trains, firepits, etc.
Team composition will be important in this game because of the different hero archetypes. You might not want to go 4 healers or 4 assassins but find a good balance of heroes that complement each other.