I'm anti-customizing (I've seen a couple people here who share my same purist attitude toward collecting) so I'm not gonna get into all this stuff about replacing parts but I'm convinced that Mattel is going to do the 4 main characters in the 12" scale and leave it at that and I've noticed that there are current collectibles on the market that are more or less compatible with the 12" Mattel figures. As always: sorry for the crappy cellphone pics:
They're the Diamond Select banks. They also have a standard "nice" Stay-Puft bank (which I also have) and there are obvious scale issues but they work more-or-less. Stay-Puft is NEVER going to be in-scale with ANYTHING. Even if they did like a mega-scaled 3' figure like those Batman and Superman movie figs they made (and those WERE Mattel now that I think about it) and you used the Minimates the scale would still be off, plus the bizarre comparable scale should go down pretty easily for anyone who grew up on the Kenner figures. As for Slimer: even though he's on the opposite end of the spectrum (a bit big) the nature of the character makes the size issue a bit of a non-issue. Also, even though the Slimer bank is clearly styled after the first film you could even make the case that it's in-line with his size in the sequel.
In any case, I'm not holding my breath for the non-Ghostbusters characters to be done in this scale by Mattel. DST is also doing a logo bank which would also look pretty good with these figures. I imagine it's probably pretty close to the size of the firehouse sign in this scale.