I thought that this would focus entirely on PC (and/or home computer) games, not console ones... So someone's Mario rip-off seems out-of-place. There's only one such ripoff I allow, and that's id's Dangerous Dave. (the original alpha, a tech demo of John Carmack's scrolling engine, was called Dangerous Dave in Copyright Infringement, and had a level ripped directly from SMB3)
While X-Com was significant as well, it's blasphemy to suggest it alone brought TBS to PC gaming, and ignoring the major 4X franchises. While perhaps not so many might like to play the original Civilization compared to the newer installments, one should never forget SimTex's two franchises, Master of Orion, and Master of Magic. The latter, in particular, perhaps tops the list of "why hasn't this game gotten a sequel yet?" After all, numerous series sprang up simply to try and clone it, such as Age of Wonders and Heroes of Might & Magic. (almost all fantasy strategy games take at least strongly from MoM)