Halo has plenty of material between the games, books like the Fall of Reach or Contact Harvest, or even events relativelely unexplored in general like the interplanetary wars that I definitely think could be made into a great sci fi blockbuster, and you can even have a potential trilogy or saga depending on the direction you decide to go in. It does have a tv series that is stuck in development hell though, and the other live action attempts have been less than stellar imo. Still, if done well, I see no reason Halo couldn’t be made into a badass movie.
Mass Effect can not only be adapted into a trilogy based on the games, but there is also plenty of lore like Halo that filmmakers can easily draw inspiration from.
Fallout would be awesome as a mini series that told various tales across the wasteland, perhaps in areas that we haven’t really been able to explore in the games yet.
Star Wars : Knights of the Old Republic would be fucking awesome, and can easily spin off into a larger franchise of movies.
While there has already been a shitty movie adapted from it, I do think an adaptation of the new Doom, made into a balls out, non stop action movie, think Mad Max Fury Road, could make for a fun as hell summer blockbuster.
Dead Space can be made into a great horror movie, either more action horror or traditional horror can work if pulled off correctly.
I’d love to see what could be done with the Dishonored license. The setting alone is filled with potential for a movie imo.