I would make a game like Anthem, but good. I loved many aspects of that game: the exosuits, the co-op focused gameplay, the open world, and yes, I like looter shooters.
Other things I would do: -Combat/traversal would be a bit more ground-based. There would be jet-assisted jumping and maybe limited flight, but not sustained flight -A modular weapon system. Take a base handgun/long gun/launcher/etc and modify it to be rapid-fire, scatter shot, sniper, target-chaining, or whatever else. -A post-apocalyptic world with a harsh environment that requires vehicles or exosuits to get around in outside of established towns/shelters.
Other game Ideas I had: -A single-player story-driven game where you play as a battlemage in your nation's military. The setting would be somewhat modern-day with magic. Due to the presence of magic, guns would be extremely limited or nonexistent, and I think would be an interesting idea to explore. -A gacha game without the gacha. I like games like Genshin Impact with the open world, exploration, characters to collect, team building, and regular content updates, but without the predatory monetization and focus on dull grinding and daily-logins to drive engagement. I'm not sure how feasible this would be without the amount of revenue a gacha game brings in, though. MMOs kinda do this, but I would want to build my own party, rather than play a single character.