I don't see why people are so against the idea of Khan returning considering that the 2009 film brought back and recast the original series Characters and created an all new universe to play in.
The original characters are essential to the reboot. But Khan isn't. And as you say, they have a whole new universe to play in. I'd rather play in new parts of it. The whole strange new worlds, new civilisations bit of Trek.
So what's the problem with bringing back one very popular well known Villain and recasting him?
There's nothing 'wrong' with it. It's just not my preferred route for them to go down. Obviously he has to be re-cast - Montalban can't play the role. I just think that Star Trek has so many storytelling possibilities without revisiting someone like Khan.
So many people who supported the 2009 movie used the Batman analogy for the restart of the franchise and recasting characters. Ok, well look at the Joker then and what Heath Ledger brought to the character.
The Joker is Batman's arch nemesis. There has to be a Joker in every version of Batman, be it Romero, Nicholson, Hamill or Ledger. He has to be reinvented for every generation. Khan was intended as a one-shot villain, who was brought back to very good use by Harve Bennett and Nick Meyer. He's not the same as The Joker.
Khan is iconic to Star Trek. He is a Villain that people know more of than even the Borg because when TWOK came out it was only the second Star Trek film to be released and there was still only one series and no over saturation. Star Trek was still attracting a more regular audience compared to today so he is one of those Villains that lots of people know because it's the movie where Spock dies. It's a classic to young and old. Why not bring him back? How many people have seen "Space Seed" compared to TWOK? There is a backstory that many regular film goers don't know and this next sequel has the chance to introduce a character and show why he would be so pissed off with Kirk to begin with. Heck yeah i'm on board for this one.
While I like TWOK an awful lot, I do think that Trek fans overstate its popularity and status among casual cinema fans. I don't think Khan is all that iconic. I suspect Montalban is better known for Mr Roarke than Mr Singh. And the Klingons are much better known than either Khan or the Borg.
As for the Klingons? Been there done that to death.
I can't see how you can possibly on the one hand argue that we need to see more of Khan and then say that the Klingons are done to death. The Klingons haven't had a prominent role in a Trek movie since TUC (the Duras sisters in GEN were supporting cannon fodder). Yes, I know they appeared a lot in TNG and DS9 but the casual fans who will see the next movie won't have seen this. We haven't seen 23rd century Klingons (apart from in DS9's Trials and Tribble-ations) since TUC in 1991 - 20 years ago.
The Klingons are the definitive bad guy race in classic Trek and Kirk had three memorable opponents in Kor, Kang and Koloth. I'd argue that if any classic Trek villains are ripe for recasting and reinvention, it's any and all of them. I think the Joker/Batman comparison is more apt here. We got the origin movie out of the way - now it's time to reinvent the hero's best known enemy.