The most recent NPD info suggested that Dragon Quest 8 was on track to sell even worse in America than Dragon Warrior 7 did (which is what Enix considered bad enough to fire an entire regional division), despite their heavy desire to see the game do well.
Since DQ8 wasn't a "Final Fantasy" level success like Enix wanted, the PS2 remake of DQ5 has most likely already been canned in America. But the PS2 and DS are entirely different markets, so it remains to be seen if Dragon Quest can do well on the DS.
Enix might (and by all accounts should) release a game on the American DS in order to test the waters, but I don't know if they'd want to try something as "unusual" as Slime MoriMori, or if they'd prefer something more traditional. Unfortunately, there aren't any traditional Enix RPGs on the DS in Japan for them to choose from yet (that I know of). But if Enix never goes "traditional" on the DS in Japan, and would rather play around and have fun with things, then Slime MoriMori would be the best choice. I think we just need to wait a while to see more of what Enix's plan for the Japanese DS is. And with the DS dominating in Japan, you can be sure that Enix does have a plan for the Japanese DS.