Why? IT seems like they woudl get the most money by doing this. 

^no,first of all we will see how DQIX does on DS,if it will do better then the console ones,but i think they would make the most money if it was released on wii,cheap to develop and its games cost 50$ as opposed to DS's 35$,which would make them more money

Nice remark Montana Hatchet, distinguishing the JRPG and the WRPG. It should be clear by the thread's title. Cause, 360, has something bigger than FFXIII already. It's called Μass Effect Trilogy.
On topic, no, I can't see anything bigger than FFXIII, saleswise or quality wise.

 Wait what? You say that the mass effect trilogy is bigger than FFXIII then say there is nothing bigger. Personally I disagree, the mass effect trilogy isn't even exclusive, and in no way holds up to the massive series that is final fantasy. Maybe in quality it compares, but in no way does it compare in sales.

 What's stopping SE from charging 50 dor the DS DQ9.  Don't they already charge like... 40 per DS game.

Well... It will look great.

Otherwise... lets just say there have been many RPGs released i would put above recent FF games.

 Lol that is what FFs come down to now. Will it look good. Hopefully XIII breaks the decline the series is in. If not then I could see the spin-off Crystal Bearers being a better game. 

Quality wise, there are plenty that could be. Mistwalker certainly could, as their first two offerings show a lot of potential.

In terms of sales, no, there's only two series at that level FF and DQ and I don't see the 360 getting either. Kingdom Hearts it could get because of the series US popularity, but clearly that series is not as big as either of others.

MS has the money to do it, but i don't think it is a good idea since the idea of buying an exclusive is just to show how much software your cosole can move, so other companies look at you as viable port to make money.