thegames might be a good fit for you! It's set in Panem from the Hunger Games, and characters are "reaped" to participate in the "Neverending Quell". Basically, every other month or so, characters are sent into an arena to fight to the death and are then resurrected in the Capitol. There's an over-arching plot with rebellion and regular events. The game is solidly medium sized and paced and we're very backtag friendly. We have a large and very active MCU cast with Steve, Tony, Thor, Bruce, Natasha, Sif, Loki, and Bucky, along with active 616 characters like Pixie, Loki and Black Tom, and Professor X from XMFC. We're having a Test Drive Meme right now, if it strikes your fancy! If you have any questions, I'm happy to help!