I think T6 ships, rather than being granted from a standard ship requisition, should involve raiding an Undine shipyard.
After acquiring the Undine ship (which is defined by cruiser/escort/science and special ability), we can customize it to take the form of any prior ship. Simply an extra drop down at the customization screen that allows us to select any prior tier, with the caveat that cruisers choose from cruiser skins, escorts choose from escort skins, etc. And then customize it as normal after selecting the tier and type.
This works because all T6 ships would be shapeshifting vessels that can take on any configuration we want, either any T1-5 appearance or its native Undine form.
The crew compliment and stats remain intact without regard to appearance because, using fluidic space rift generating technology, these ships utilize tesseracts (similar to the TARDIS in Doctor Who) and the external appearance can be set without regard to how large the interior is. (Thus, a Connie or a Defiant can have upwards of a thousand crew onboard and the one size fits all interiors are not a storytelling issue, nor are the Undine-sized interiors.)
Naturally, because these Undine ships are "blank" when we take them, we can only program them with our own faction's ship appearances because we do have to input the schematics and specifications.
omfg..t6 are like half+ year away and all already whine about lack of canon ships and now that...t_t there no plans for rising level cap and t6 ships yet from reports we dont even get our 5.5 yet.
I'm not saying these need to be in tomorrow. But I'm explaining how T6 can visually allow for all prior tiers.
This would allow people to fly ships that LOOK LIKE Connies, Defiants, Galaxies, etc. and perform equally well. This creates visual diversity in ship battles and allows for ALL canon ships to be represented within a tier WITHOUT having to manually create a refit for each ship class.
You have one ship that can take on any form. If size issues are a concern, we know that Undine ships aren't necessarily to scale when they shapeshift so T6 ships could cosmetically look like existing ships while being scaled up for visibility issues.
How about this be given to klingon players first or only, it wont make sense for the federation to steel a Undine ship only to give it strate to the captaine responceable for capturing and bringing it back, as the Feds would want to send it to memery alpha to reserch its tec and desine, in order to then aplliy this to there own ships.
Heres how the feds might do it.
The Feds could get a kind of mixst tec refit for the ship that you used to compleet the mission, but only after doing a memory alpha related mission that involvs takeing a proto type ship on a test to see how well the Undine and Fed technologes work together, as there whoud be troble with combineing two vastly diferent Techs.
It might not be much tho so it could just be a damige resitence to the ships hull and shilds aginst Undine, and a damige bounose, and some other bonose related to your characters profeshion, that whould depend also on ship type. It could just be cosmetik like a new materall in the ship desiener, called Matereall: Undine, and give your ship a golden brown collure, with an apropreut story akolade to unlocke it.
The klingons could do it the way the OP describd, whith apropreat missions and akolades, tho they'd imideitly send it to battle and win the war, I think, after geting rid of the borg that is.
Also from what I read the Klingons need some thing like this and others to keep theere player populaion up with the feds, so they shuold get this way befor the Feds, if the feds should get it at all.
This is a grate idea for kingon content. A vere good idea it is from the OP.
I think we're ok with the ships that we have right now. Granted I do want to see the introduction of new cannon ships which are coming with S2. But, as far as a new tier, I think thats a little farther off.