Can people make mods for this game?
Most people who play Civ-style games have their own views on how the game should be structured. Civ games have always been pretty good at allowing the user to control that, and that creates a loyal, heavily involved fan-base (this site is living proof!). It would be great if this stuff wasn't hard-coded, and people could tinker with the rules, units, terrain, tech-trees, wonders, buildings, etc. I think that any game that seeks to be so true to the Civ series (as yours clearly is) without the ability to tinker with the inner workings will have limited appeal to Civ fans.
The flip-side of that, though, is that you can market this as being simply one massive, well designed stand-alone mod for Civilisation/Colonisation. This might be a better way to go, but only if you're willing to put it out as shareware, since I doubt people are willing to pay for a "mod", regardless of how good it is.
One thing... Your tech-trees, units, and general game concept is heavily based on Civ. Make sure you're not breaching any intellectual property laws by releasing this...