While I do agree it'd be cool, you'll all letting that blind you to gameplay. A nova would NOT contribute to gameplay, it would only harm it. And if theres one thing that remains constant within the modding group, one thing that comes before all else. It's gameplay. So the the nova should not, and more then likely, will not be added.
I mean Heaven gave the most extensive limitations but lets go through those and see how little they'd actually do.
lvl 8 research in military and civ? Ok, I get that EVERY game I play.
Increidlbe cost? Meh, I put a novalith on every single planet in the late game, cost means nothing to me
Only the Marathon can launch it? So? By that time my entire fleet is composed of Marathons, that's like a hundred Nova bombs just sitting there
10-20 minute cooldown? As said earlier, if a Marathon launches it I allready have a hundred nova bombs, I don't need to worry about any cooldown.
Full anti-matter research? Again, by the late game I've maxed out all tech anyways, so no problem.
A nova will never work in Sins, ever. It's like adding shields to the UNSC, it's not going to happen.