It would be interesting to see a roleplay in which the players really are weak and nearly defenseless against the monsters. I think numerous and varied monsters with characteristics ranging from the deadly intelligence of the xenomorphs all the way to the sheer power and size of the titans roaming a world populated by sparse human survivors trying to last another day in any way they can would be interesting.
Perhaps two to three known species of creatures with various sub-species could be implemented to prevent things from getting stale with the same enemies, but also controlled so new monsters don't just get created left and right.
I like the idea of a futuristic world brought to ruin in some way or another, so advanced technology is present but not always common or in working condition. Seeing people play as cyborgs, androids, or straight up robots alongside regular humans would please me personally.
The players could have their own personal drives and goals, while a good overarching mission could be to find a sort of "kill switch" for the creatures, a working vessel that can carry them away from the planet, or perhaps a city/location that is fabled to be impenetrable against the monsters.
Having it written that "the end" arrived around twenty years before the role play starts would be a good idea, I think. Characters with multiple perspectives would be present, with some having been born into this world, while others would either have been very young when it happened or perhaps have had their own lives going for them before it was all ruined.
A good system for worldbuilding would be to allow people to write (within parameters you set of course) their own locations, technology, people, and cultures into their backstories, giving you a plethora of lore and content to work with when managing this world for everyone.
There you go, plenty of suggestions for you. I say why ask questions when you can create the answer yourself. :)
@Marshall