28 to 30 characters could cut it, considering that there are 13 characters alive after MK9.
(14 if you count Skarlet)
IMO Sub-Zero
(Human or Cyborg, but only 1 version this time), Liu Kang and Kitana should come back, either in story or the Krypt.
Maybe not Kitana as much, but Sub-Zero and Liu Kang are definite must-haves for anyone that wants the OG cast back.
MK: A was a rush-job, yes, but working on THAT many characters in the allotted time was the issue.
I agree that NRS has more $$$, resources and time this go-around, but I still don't want to see a 60+ cast.
I think that MK10 should have some of the MK 4 thru MK

A characters, and save the MK

thru MK:A characters for MK11.
It would be the new era MK trilogy (MK9, 10 and 11) in a sense, and NRS could even possibly market it that way too, as a collection in the future.
Now, using my example in the paragraph above
(this is just an idea, so please don't attack me all at once):
13 characters are still alive, as of MK9.
One idea that I have is that Sub-Zero, Liu Kang and Kitana come back, which would add 3 dead characters to MK10.
That would be 16 characters, and going by the approximate 28 character rule, 12 slots would still be available (not taking any hidden characters into consideration).
The remaining 12 to 13 slots could consist of
Fujin, Reiko, Tanya, Kai, Kenshi, Bo Rai Cho, Li Mei, Mavado, Nitara, Frost (with a major revamp), Jarek (with a major revamp), Moloch and Drahmin.
That covers the MK4 thru MK: DA era for this game, and would = 29 characters (Oops! I went over by 1 character), considering the other characters that survived at the end of MK9.
New characters can be introduced via DLC, just like Skarlet was in MK9.
I don't think the dead characters should come back in story mode.
I said that if they did, it would be a + for me, but I agree 100% that they shouldn't play a major role.
MK 9.5 isn't what I want either.
My example above would be a good way to include SOME of the MK9 kast, while introducing a new cast for MK10.
For MK11, a handful of MK9 and MK10 characters could be there, but no more than that IMO.
A good majority of the MK11 roster could consist of the MK

thru MK:A kast, rounding out the new era trilogy.
NOTE: Before anyone reads it and attacks me for it, this is just an idea and I never stated that this is the ONLY option that NRS should go with, just 1 OF MANY options that NRS could go with