Personally, I would appreciate a XIII-3. I've seen the ending (and secret ending) of the story of XIII-2, and am of the opinion there are way too many threads left hanging for them to not wrap things up. As I recall, Final Fantasy games tend to leave very few, if any, such threads by the end of the story...probably why the VII games and films are lumped together, so they can tie off all the remaining loose ends, no?
There weren't a lot of lose ends at the end of VII at all. Square only made so many games/ movies/ OVAs for money; to the point where certain events had to be completely ret-conned or contradictory. Sephiroth was originally killed by Cloud, but in the Last Order they made it look like he jumped himself, so then
another game had to come in to rectify that.
Now all of Square's games have incomplete endings, so they can make sequels for some quick money. As the story is needlessly dragged on, the quality goes down.
As for FFXIII Versus...truth be told, wasn't on my 'top list of expectations'.....I've dabbled in my share of Dissodia, and not so big a fan of Final Fantasy fighting games. In my honest opinion, Final Fantasy are RPGs and should stick to that game type.
Versus isn't a one-on-one fighting game, the battle system is more similar to action games, like Kingdom Hearts. Birth By Sleep has a really good battle system, but I also like the FFXIII battle system. Currently, action games are gaining popularity in Japan, so the developers follow suit.