I spent more or less all night trying to find a game to suit what I wanted to play as a tower defense game. Well, I think over the next few days I am going to attempt to put together my very own miniature game system. I have a pretty good idea of what I want the system to play like, and how things are going to work, it just comes down to actually doing it and writing it down in a coherent manner. I have never really done such a thing on my own before, so this will be a learning experience.
Another sticking point for me is how to handle the setting. The inspiration for the game came from "Orcs Must Die" and does seem to be the mode that I am more interested in using it for. However, there is no reason such a system would not work perfectly well for Terminators vs. Genestealers, Cogs vs. Locust, GIs vs. National Socialist Undead...you get the idea.
Currently I am undecided how to handle this. I can either just write it with Orcs and the portal/castle background then make a note about using it for different settings, or I can write the rules with generic terms and leave the players to make up their own background. I have seen games put together both ways, and I personally never cared wither way when I read them, but I don't know if there is a general preference either way in the community.
As I get this system put together, I will of course share with the community.