Ambassador, Oberth, Excelsior, and Nebula all seem to be quite popular ideas.
Definitely. Considering the Ambassador already has two canon variants, the Excelsior has two variants plus a kitbash as another class ( Curry) & the Nebula has around four variants, they would already come to the game ripe for customization.
I also want either the add-on pods to turn a Miranda into Sisko's Saratoga or all the extra canons to make a Soyuz class.
Also, I would love to see Picard's old Constellation Class in with the Heavy Cruisers or the Vesta Class from Pocket Books to go with the Titan.
& since we got the Wolf 359 inspired Cheyenne class, I would love to see her sister ships the Niagara, New Orleans, Springfield & Challenger classes.