Yea, it was bound to happen after a while. Two major things as I see it:
# The game needs to be refilled with new assets and such, but it takes time for us to do these things since we're only two people. Even if I have two artists that helps me with some things, like the concept art backgrounds and bosses, I need to manage them. I woulden't mind having a third person on the team, a graphical artist probably, sitting here with us.
# It's to time consuming to make levels for modders, just to make a simple thing take time and then you can't duplicate it easy enough over levels in a campaign, you can't share it with other people good too. Some people have started some big map projects, but I know they will never be able to complete them, it just takes to much time and work to do it properly.
