To avoid copyright issues with games you charge for, you should:
-> Avoid using an assets or game code
-> Avoid direct naming of a product (brand name stealing = bad idea)
Generally if a game is free to play and doesn't cost someone anything to use it, someone like Activision won't care, if you use the game assets. However, if you charge something, they may take interest in it, in a bad way. Like saying that the game is technically their's even though you built it (considering they own the franchise and brand name), and if you argue over it they'll sue you for it. They may however instead just take royalties from it, and leave it in your hands. However, if your game damages their reputation or brand imaging, you may get sued heavily.
So...
If you intend to offer the game as a paid item, you should use your own created assets, code and storyline, as well as change the name of the product to something else. You could say that the game is a "spiritual successor" to Outpost 1/2. Similiar how Uber Entertainment is saying that Planetary Annihilation is a "spiritual successor" to Total Annihilation (which I believe they don't own the franchise or brand name for).