The following are the correct 1-syllable contractions for 무엇 and its subject and object markers:
- 무엇 = 무어 = 뭐
- 무엇 = 뭣
- 무엇이 = 뭬
- 무엇을 = 뭘
Unfortunately, it is probably too late to save the 뭬 contraction because 뭐가 and 머가 have become so ingrained in the Korean language that it would be difficult to get rid of them now. I think I remember reading that sometime in the past 가 used to be the only subject marker in Korean, which may help explain why 뭐가 is so ingrained.
By the way, if you do use 뭬, do not add the subject marker 가 to it since its meaning already includes the subject marker. Some Koreans even add the object marker 를 to 뭬, which, of course, is also wrong since it would be like saying 무엇이를.