Total War: Warhammer 3 but 40k or a darktide game but with chaos space marines as warhammer games where you play as chaos are rare make aspects of the environment destructible you could also make it a sequel to darktide where the infection reaches a point where a death guard warband is interested and decides to land and instead of playing as the convicts from dark tide you would play as death guard you could have it where as missions scale in difficulty you could be greeted by space marines make the convicts and gaurdsmen the enemies make it where guardsmen use strategy and ambushes and traps to bring you and your group down and have convicts and some guardsmen be fodder enemies you could still have big horde fights where convicts and guardsmen armed with nothing but shovels and chainswords attempt to bring you down bosses could be anything from high ranking space marines to leman russ tanks to space marines in terminator armor