There are three modular DN designs for the Romulans in SFB Module R7, though none of them are yet in FC so far as I'm aware.
The DemonHawk would have had three engines (each of which doubles in size relative to those on the SparrowHawk). Its modularity would have come from a pair of SparrowHawk modules and a single SkyHawk module slot. The "REDHawk" is a DemonHawk equipped with both SparrowHawk-E and SkyHawk-C PF Tender modules, which together would have made it one of a very select group of ships permitted to carry two gunboat flotillas into battle.
When that concept was passed over in favour of the Condor, the same designers came up with the MegaHawk instead. It would have had six SkyHawk/SparrowHawk engines, with two pairs of SparrowHawk modules (one set hard-welded, the other set swappable). Two keels were laid down, one in Y182 and the other in Y183, but the money wasn't there to actually finish either of them by then.
As it happened, one of the two MegaHawk keels ended up being completed as the less powerful OmniHawk design. With only five SparrowHawk engines instead of six, the ship was more of a "light dreadnought" in terms of size (but was not treated as a Fast ship).
Of the three, I'd probably sooner want to see the DemonHawk done for 2500, if such a ship were up for consideration at some point.
But it might be worth noting that other empires have modular dreadnoughts of their own. Over in the Iridani Cluster, the locals' Quest ships are, for the most part, designed with modularity in mind. Their Brigantine dreadnought fits three modules, while their Man-O-War battleship has four. (SFB allows Iridani players to mix and match these module selections for themselves, but the playtest Ship Cards for FC offer pre-prepared configurations instead.)
The Omega Octant's Bolosco Merchant Guilds (curently only in SFB) operate a fleet of tugs, and their mercenary DN can fit two small pods to allow them a degree of flexibility in terms of which role they were to serve in.
Plus, one of the "lost futures" Omega factions (those which are known to exist, but which have yet to be formally published), the Zosman Marauders, are said to operate a wholly modular doctrine of their own. According to the article in CL36 describing Brice Graw's original vision for them, their Godfather DN would fit no less than six modules (as opposed to four on a CA, or one on a frigate). But it may be a while before we see any Zosman ships committed to print.
