Any Cr card has an equal chance of becoming an Mt card.
Invest in Soleil or any Cr. You might be lucky, you might be unlucky.
I personally think staff might pre-plan a Cr to Mt in the case of Nemo Mt if memory serves and special comics cards (Cannibal Jo Cr and Xantiax Robb Cr) are said to have an increased likelihood of becoming a Cr shortly after their release
Although we have been told that any Cr could become Mt, I particularly think new Crs that are not specially realeased cards (Such as Oon Cr or Pr Cushing Cr ) have less chance of becoming Mt.
I think we can say that there's still a priority related to the period of time those cards have been in the game, a.k.a., "old cards" become Cr and old Cr become Mt. This doesn't apply to specially released cards, since they are, well, specially released.
Spyke was, technically, an "old Cr". Even if he hadn't had too much time as a Cr, he would only be 1 out of 10 Mts we've gotten so far that was not old enough or specially released to become Mt, meaning that it's highly possible that the next Mts would be really old Crs or specially released cards.