I think the sequel could be about the core being rebuilt and the mobs returning but just more mutated and maybe then new mobs too.Some mobs could even have buffs like some specific gun can’t damage it. The maps would be in another lab but this time in a major big city and when the core is rebuilt the city gets overrun by the mobs.
this is just my idea If you would like to use it. (Edit: I got a question how did u get the idea to make this game?)