For my part, my current line of thinking is that there are three levels of pre-existing SFU design, in terms of what stage they are at, and where they can likely go:
*The hulls which are directly drawn from the various source materials (both on-screen and in the Star Fleet Technical Manual); the Fed CA, the "Franz Joseph" designs (as and when they appear in Starline 2500), the Klingon D7/Romulan KR, etc.
*The units which were created for use in the SFU, but which themselves have established a long-standing presence in game and/or miniature form; the Fed NCL, Klingon D5 et al.
*The ships from eras or settings which have either not been given much in the way of "legacy" art, or which have no miniatures to speak of; the early Vulcan and Andorian ships, most of the non-Alpha empires, and what have you.
Categories 1 and 2 are the ones that, by theur nature, are the most hotly-contested; not least because they are the ones most heavily integrated into the established logistical networks operated in the likes of the strategic game Federation and Empire. (F&E is mostly oriented towards the General War; though there are a handful of scenarios for earlier or later conflicts, such as the Four Powers War and the ISC Pacification Campaign.)
But even then, there have been at least some changes, albeit more subtle ones. The SparrowHawk and FireHawk are good examples of this; indeed, the FastHawk had never been given an official miniature before, and I think the design turned out great. And even with the more iconic "category 1" minis, the versions we see are not quite the same either; I've seen complaints on this forum that the D7 is not the same as the TV model, which is ironic considering Mongoose were more keen to make it look different. (But then, even a change like that made to the Fed dreadnought's secondary hull has been justified as being part of the conversion process to make the DNG; which could leave the door open to a "classic" Franz Joseph DN to be done for a 2500-series edition of Squadron Box #91.)
The thing is, though, that the very nature of most category 3 designs means that exploring them would have to wait until more of the Main Era was fleshed out first. Ships like the old Vulcans and Andorians, and the rest of the warp-refitted Terrans, would only really work if a proper, separate Early Years setting was put together for ACtA:SF at some future point. (If you look over towards Victory at Sea 2.0, I'd imagine that doing pre-dreadnought minis would only work if/when the bulk of the WW2-era fleets are up and running first, and would also depend on whether or not there will be a VaS 2.0 edition of a rulebook exploring the pre-dreadnought era.)
As I said, to get there, the "hump" (of dealing with the more contentious designs) would have to be dealt with first, before the door would be more widely open for something truly new in miniature terms; and even then, it would only work if people are willing to treat each new setting on its own terms, and not use them as a means of trying to shoehorn new designs into the Main Era.