SFC was based on SFB, and FASA designs have become mainstream in the SFB world, but, let us not forget that SFC is a game all its own, and therefore, I agree with MP that a brand new design should be made, however, I don't agree that the new ship shouldn't have design elements from older, more tried and tested designs, which have stood the test of time. I'm not saying oversized, bastardized bashes. Just new designs, with hints of older ships in their design philosophy.
In canon Trek, the Feds never had a BB, and DNs were viewed as a contradiction to Starfleet's very nature of peaceful exploration, at least from what I have read on the subject in the past.
I do not agree that the Excelsior was ever a BB at anytime, or even a DN. BCH, yes, in size and firepower based on the standards of the time, and the Enterprise B would have been an improved BCH, and by BCH, I mean Battle Cruiser/Heavy Battle Cruiser.
It is evident that as technology progressed, and tensions within the galaxy always changing, that Starfleet, just like other races, built bigger ships with more advanced technology. I do agree that the Ambassadors were supposed to replace the Excelsiors as the frontline explorer in the fleet, but it didn't happen, but, they did take over as the battle cruisers in the fleet, which put the Excelsiors in the role of Heavy Cruiser.
It is believed that the Ambassador production was cut short of its intended goals. I don't believe this to be true, personally that is. They are more than likely still being produced, just not as fast as previously intended. But I am starting to go way off track here, so I'll try and get back on track.
I do believe that if there was a BB in the TMP era, she would be bigger than the Excelsiors. Maybe she would dwarf the Ambassadors but only a tiny margin. She would be a completely new design, but share design traits with those of the Constitution-class and the Excelsior-class. Reason for the Connies is that at the time they were the most successful design ever created, and the Excelsior-class because, short of the transwarp drive, the Excelsior was a very stable, viable design, with the systems designed for her being adapted to the entire Starfleet, refitting all current ships in service.
Just my thoughts.