a retro fit is possible for every ship class if you base it off your capt teir. as say a capt teir 4 you have X number of console slots and ship slots and you decide what they are going to be ie... 5 in engineering 4 science 3 tactical.. 5 forward weapons slots 2 rear extra..
As well all bridge officers are used not just 4 or 5 . So a larger ship would have more bridge officers spots.
than a defiant or bird of prey.
Or capabilities. It would be easy to make a tier 5 Constitution if you just used the outer design but made it bigger. Maybe it technically wouldn't be a Constitution anymore; but it would look just like one. Bam, you have a Galaxy sixed Connie.
Of course, it's only game mechanics that say only an escort can mount cannons, and we already have exceptions to that; the battlecruisers and Galaxy X can both mount them. So you should be able to make any ship mount cannons through a refit. The idea that a Defiant can mount more powerful weapons than the biggest ships in the fleet doesn't sound reasonable.
Just make the ship bigger, call it the new Akira, Constitution, whatever, and you can have any ship be tier 5. Once we are Fleet Admirals you'd think we'd have the ability to make our flagship be anything at all; after all we would be the people who decide what ships get built next.