Doesn't need a sequel, he can add so much to this game if he wanted to, classroom job, infirmary job, training job, cafeteria job, cleaning job, list goes on, and he can add tons of new dungeon layers with new enemies, game is option wise very easy to expand.
Problem is the dev is tired with developing further, he just wants to make it perfect and start something new, understandable. However, if I sat on this goldmine, I'd expand the hell out of it. Better to expand on a golden egg than risk something new which could be a dud.
Hopefully once he claims it's finished we will have some awesome modders going Skyrim on it.