You know, an Akira Retrofit would fit thanks to a simple argument. We already have the Excelsior and the Nebula at Tier 5, why not the Akira? Following the tradition of those 2 ships, the Akira would be a Fleet Escort with a Lt. Com Eng station. Simple.
As for Cryptic designs. I actually like the C-Store Star Cruiser + Emissary saucer. I'm also trying to find a good looking Fleet Escort since it is my favorite escort layout, but everytime some little details screw it up, often those little wings on the top. I whish the design was more towards the Intrepid Type from Enterprise. The newest C-Store ships are nicely modelled, but I have to say, I hate the T2 Escort Refit, this ship is just plain ugly.
The fleet escort has a certain ... rugged build about it. If I hadn't bought an MVAE, I would still be flying the escort, dervish nacelles but a more gryphon look overall.
Of course, the MVAE is a pretty piece of material ... in canon, it would have begun regular combat duty shortly after the end of DS9; indeed, one is flanking the USS Voyager when she returns to earth at the end of Endgame, and Admiral Ross selected one as his flagship in some non-canon novels. The MVAE continued to be in service until the 26th century in an alternate timeline when Archer visited the future. There's nothing wrong with seeing a lot of those.
There are problems with seeing a lot of Defiants - from what I can figure, that ship tears itself apart and requires the very best starship engineers in order to keep the ship from blowing itself up. Case in point - a bunch of cadets couldn't get it to go faster than warp 3. However, the knowledge of the class was well retained into the 31st century, so its plausible that the class be retained ... although I wouldn't have it be one of the top tiered ships in the quadrant - such distinction should remain with flagships, not tactical escorts.
The defiant also has tons of sizing problems, from terrible deck plans, not actually having enough space for all its weapons, impulse drives, and shuttlebay, airlocks (used with ds9), windows (which it shouldn't have any at all) .... and so on.
Or they have done revisions to the desing in the 40 years that made it a little more viable, as they did with the Excel'...