I thought about it and to be fair, almost every other XM character is not nearly as popular as Magneto, or there are very popular characters already being done and done (like Spidey). The other possible Magneto2.0 were Phoenix and Thanos imho, but they missed the mark, one with the heads, the other with the base.
As for Ghost Rider... he's great but I got the impression that a lot of people were mainly attracted to it being an exclusive, plus with the high retail price + high markup, the room for a high secondary value gets smaller and smaller imho.
As for Cable, not sure how reliable it is as an info, so take it with a grain of salt, but a few resellers I spoke with all told me that the demand is crazy. Even at the inflated rate of 1250-1300SGD they sell it for, they have more orders than they can reasonably handle.
By the way I agree that it will be much harder to see anything appreciate in value like Magneto did, for a lot of reasons. One of which is the increasing costs, retail prices are going up, markup by resellers is going up, shipping rates are going up, more companies are offering a wider range of characters in various formats and scales... I remember I paid roughly $800 delivered for Magneto, Thanos was 900$, Phoenix and Black Bolt 1000$, if I picked up Cable now it would probably be around $1300. I'm interested in Weapon X but that too would end up being somewhat above 1000$... looks like a trend, and I'm not exactly comfortable with it.