For what little it's worth, I'd be somewhat reluctant to see too much spillover from the Early Years ships into the modern era; for one thing, I feel it cheapens the W- and Y-eras, since it implies those settings aren't worthy enough in their own right, but have to be shoe-horned into the General War in order to work.
The whole point about the Fed CL is that is was, as old armourclads went, an odd duck; no-one working on the early sublight versions had any clue that the ship would end up being so enduring, as there was no reason to expect advances in warp technology to enable it. It just happened to work out that way. (Just as it also happened that ships that were viable for their era, like the old Terran heavy cruiser and frigate, ran into technological and logistical dead ends instead.)
And the Romulan case is also an odd one, stemming from the technological handicap they were hamstrung by for such a long time. By the time of the Treaty of Smarba, they simply had no choice but to heavily invest in upgrading the Eagle-series ships in order to catch up with their rivals. (And like with the Fed OCL, the progression from Warbird to War Eagle to King Eagle was far from pre-planned in-universe.)
In the case of the Klingons, yes they were reluctant to throw away a weapon (or a starship), but the old D3s were really starting to get worn out by the time they were supplanted by D4s; and especially when those later ships were in turn relegated to the Klingon naval reserve by the onset of the modern D6. Plus, it is noted that many of the surviving D3s had been assembled on the Romulan border, and were set to be used in the planned conquest of the Romulan Star Empire (who were still stuck at sublight at the time, and were essentially defenseless against a fleet of even these warp-powered ironclads) before they were destroyed by the Tholians in the establishment of the early Holdfast.
Even if there were surviving D3s that weren't fit for mothballs (or the breakers), they were simply unable to keep pace with events; they were outmatched by Y-series ships (to include the first Orion YCRs and YLRs) and were completely outclassed by the Holdfast Tholians (whose PCs were, essentially, the X-ships of that early time).
In the case of armour, there are two kinds of "normal" armour seen in the SFU; lattices of energy-dampening crystals embedded in a given hull, or separate armour "belts" which can be removed or replenished more readily. However, almost all of the armour seen in the Alpha Octant is if the first kind; the lattices are literally woven into the hulls of armorclad ships as they are being built, and cannot be removed (though they can be repaired). It's simply not possible to add (or subtract) armour boxes from such ships, since it's too heavily integrated into the ship itself.
Also, it's worth bearing in mind that in the Early Years (and even in the Middle Years), drones were quite slow, and nowhere near as effective as they are in the General War. The Klingons didn't have anything like the same impetus to go wild with drone variants back then; it is questionable whether enough of the old hulls were still around by the time the better drones were available to make dragging them out and using them worthwhile.
I don't work for ADB, so I won't pretend I have any sort of say in the matter (and nor would I want to claim as much); I would rather see a potential D3 miniature wait until it could actually get to operate in its natural environment; serving the Empire against its enemies of the Early Years.
(Please note that I'm not trying to be harsh with any of this; but you should note that any submissions on the ADB BBS would be subject to a vetting process, especially for Alpha Octant empires that have a lot of their available niches filled by pre-existing ships already. That's not to say that new designs can't be added - they can - but they have to work with the broader setting.)
Also, of the Fed National Guard fleets, only the Terrans saw any of its hull classes covered by the deal with Iron Crown; it was the five founding species of the
Inter-Stellar Concordium (Korlivilar, Pronhoulite, Q'naabian, Rovillian and Veltressai) which had their planetary fleets exclusively based on ICE miniatures.
And the "early Smarba" Kestrel takes on the D4 and F4 were published for
SFB in
Module R8; but the issue there might be whether or not they would have to wait for the equivalent ships in the Federation, Klingon Empire, or elsewhere to be ported over at the same time. (But, again, I might argue that even if they were, it would do the hulls more justice for any would-be miniatures to be intended for use in the Early Years first, rather than being tacked on to the Main Era as an afterthought. That said, if we ever get an Early Years D4, there'd be no reason not to then offer stats for the LD4 or KD4 afterwards; so long as it was expanding upon its primary use as a D4, and not the other way around.)
