I've been thinking about this, and I think perhaps something that looks a lot like a K'tinga in proportion and shape, but just slightly more modern looking, and the size of an Excelsior would be cool. Call it a D-17 or something. I've got a couple of sketches I need to scan in for my ideas on this, but that said, when the Excelsior was new, Sulu was really not hot on the idea of going toe to toe with a K'tinga, so the size disparity means little between the two ships.

The most obvious reason why Sulu was not interested in engaging a K't'inga (if that event ever occurred, that episodes was totally messed-up nonsense) was because he did not want to start a shooting-war with the Klingons.
Besides, we saw how "troublesome" a Constitution (a ship twice its mass) was for a D7, why would the K't'inga, essentially an upgrade/refit of the D7, inexplicably be able to take on a ship about 10-15 times its mass?

i think by the 2290s the threat level of a ktinga was squarely between that of an excelsior and connie. it wouldn't be an easy fight for the excelsior, and would be more trouble then its worth taking it on, not counting political complications. that's part of why i imagine the whole constitution line was basically phased out at the end of that century, it was simply uncompetitive with its old rival and much more advanced designs and space frames were being introduced. i do think that a few were being flown for at least 70 more years as training vessels for cadets, and 1 might have got mixed up with that wolf 359 indecent, but certainly they are not in any active service like they are now in game.

Same reason the Defiant can I'd imagine. No labs, no recreation areas, just a lean no frills fightin' ship.

D7 didn't have those either.

Ah, but the D7 class lacked the most important part of a ship. A really cool name.

Not exactly, the old FASA material, which also contained the name "K't'inga" for the ships from the first Star Trek movie (also designated as "D-7M" in those materials) also contained names for previous D-7 models.
The earliest for example was called D-7A "K’t’agga" which means "Painbringer".
I think that qualifies as cool.
And while none of the old FASA stuff is considered canon, neither is the K't'inga designation when going by the strict fact the ship was never called by that designation onscreen.

Well, I searched for more than an hour, but I couldn't find the post I wanted to quote..

One of previous times a refit-Connie was brought up, something was mentioned about it not going to happen. This was not because the devs were against it, but when they had considered it Paramount had put its foot down or something. Basically I think the message had been "not going to happen".

What does this have to do with a K'tinga refit at tier 5? To be blunt, if we got the D7/K'tinga at end-game, the Federation would scream murder.

Also, to be truthful you can fly the tier 3 K'tinga at end-game. I have done so on several occasions (though never in pvp) It just requires a more careful hand and you have less abilities, lacking the commander bridge officer ones. By comparison the tier 2 Constitution is more or less unflyable at end-game, with even less abilities and bridge officer powers, not to mention being under-gunned by far. So, when it comes to flying the classics, we Klingons are slightly more fortunate.

At this point, though I love the K'tinga, originally it was what I wanted to fly, I must admit I don't want to see it at tier 5. It would be an about 200 year old design. It has been allowed to be more powerful than the Constitution class, despite this being about a 100 years younger. Indeed, the K'tinga is tier 3, just like the Excelsior (non-refit), and Cheyenne type vessels, much newer designs by all accounts.

To be blunt, it would be somewhat wrong to have a tier 5 K'tinga without allowing for a tier 5 Connie or Connie refit. As far as I know, or at least remember, that is not going to appear.

I would have to disagree with you.... the Connie and the K'tinga are two different issues in my mind, it's like comparing apples and oranges. The K'tinga is a contemporary ship still in service, it is constantly updated and outfitted with new weapons, plating, shields, engines, etc. [ Memory Alpha ] The Connie wasn't. Just think of it like the D'Kyr it only 'looks' like an older ship aesthetically.

Personally I think it's a shame they are not planning a T5 Connie, it's the one thing that would make me want to spend any time on the blue team. :p

They were however never really clear what they considered "old" ships that should not be upgraded.
Only that the Constitution was among them.
So it might apply to older ships among lower tiers.
The K't'inga is however at the same tier as the Excelsior.
So if Cryptic decided to not upgrade it (even thought they stated as much in "Ask Cryptic August 2010") they'd be using a double standard for the Federation and the Klingons since the upgraded thier ancient tier 3 cruiser as well.
If they had put the K't'inga at tier 2 (which might have made a little more sense in the first place IMO; K'Vort would have been better at tier 3) there would be no doubt in my mind that you're correct and their statement applied to the K't'inga as much as it does to the Connie Refit.
But given where they put it...we'll see.