I do think there's something to the core Rivals conceit of: Villain X achieves a power that threatens the multiverse. Now, alternate reality versions of heroes across the Marvel catalog must team up to take them down. From an RPG standpoint, it provides (1) a clear central threat, (2) reason for heroes to work in teams, and (3) an excuse to play around with Elseworld/non-canon interpretations of characters we've all written a bunch already.