The project is reportedly being shopped around to other directors, although no names are being mentioned.
Lin is currently set to direct a Hot Wheels movie, while Wan is handling Aquaman for Warner Bros. Both of those films are looking at 2018 release dates as well, so juggling two films would be tough, if not impossible.
Even if one of them was interested in directing Mulan it wouldn’t seem that the scheduling would work.
There are certainly some other names, both high and low profile, that Disney could approach.