I like this trailer. I especially like how you suggest a more modern setting for China (1800's or something) because of your usage of guns.
As far as casting goes, I think it should be a no brainer that Ken Watanabe assists, presumably more as the Emperor, and Jackie Jan might make a good comedic role (I'm thinking maybe Peng or something).
I would totally buy having Eddie Murphy reprise his role as Mushu, since that's the only voice perfect for him.
As for Mulan herself, I wouldn't be surprised if Ming-Na Wen (Agent May from Agents of SHIELD) reprises her role as Mulan because she can still kick ***, but if they were going for someone younger, I'd recommend Lucy Liu.