Warhammer: Battle March. If they'd released mod tools, there would certainly have been someone out there to release a closer to Army Book mod, which would have almost certainly draw in more Warhammer players who skipped it because it wasn't accurate. And then we could have gotten more armies modded in! A full-fledged Dwarf Army! vampire Counts! Brettonia!

Of course, then there's always that chance that none of that would have happened anyway.

You know what game could have used mod tools?

Pretty much all of them? ;)

At least someone could recreate Dark Omen in a form of a mod.

As someone who hasn't had a chance to play Dark Omen (Or Shadow of the Horned Rat, GOG), that would have been awesome. Of course, it's probably far too late to release the tools to any success now.