Personal favorite idea I had, but a riot shield or something similar would be rather interesting! My idea is it would heavily increase survivability but limit what guns you can use with it. And maybe in late-game you can make new guns that are compatible with the shield, like a smaller, one-handed version of the tesla rifle! And maybe different shields as well, like one that covers less, but allows use of large guns like the rocket launcher, or a shield with a heal/repair function built into the inner face of the shield, for even better survivability!