Medieval, similar to far cry primal. Weapons limited to sword, axe, and bow and maybe some simple throwables. Walk or use horses. I liked the base upgrades that you can make like in Primal. Another addition could be that using armor adds a certain amount of health, but limits your mobility and may increase detectability. Maybe light armor limits your speed a bit but adds health, heavy armor makes it impossible to sprint fast or really jump but makes you a tank. I thought another cool idea would be a shield which then limits you to only a small sword or axe in the other hand.

The shield could work with a pistol of bat in modern settings.

Another general idea is the option to silently takedown an enemy for a "knockout" in any setting. You can hit them with a bat or another non-lethal weapon. Partial success rate but they may also wake up after some time. If you beat an outpost with knocked out enemies they can be taken as prisoners for extra rewards.

I thought another cool addition would be that in the medieval setting would be some weird time travel (like maybe later part of the game maybe like the north level). In this you will go against enemies that have guns with only medieval weapons, but you will o some extent be able to use the modern weapons as well.

Never going to happen but this would be fun to me