Extensible open world-y shooty thing, in the vein of Far Cry or GTA games. First class mod support so game content itself is a “mod” and nothing hardcoded. Individual, potentially really small bits of “logic”/behavior can be switched on/off/out, such as different driving physics/enemy AI modes/arcade shooter vs ballistic simulation. Behavior written in some kind of fairly accessible language. Multiplayer support.

In theory it should be (potentially) everything to everyone. That never works in practice, of course, but I think it would’ve been neat. The main goal is to take a lot of the pain in the ass out of modding so as to enable more of it, make things possible that are generally annoying/impractical in modding existing games with usually dogshit support etc