I voted for Romulan War/Birth of the Federation just because Romulans are my favorite race. I would honestly be good with any of the options, especially if they at least have Romulans in there from time to time.
For a major species, their involvement is understated. TFF, TUC, GEN (as corpses), NEM and ST’09 are the only films they appear in.
Meanwhile Klingons have being involved in every film in some capacity, except BEY.
And a good thing that happened too. Because there's no way a Romulan war movie would have re-energized the franchise like JJ Abrams' film did.
There’s no way to know that.
The 2000s was still the age of the action hero. The tailend of that era, but that decade was still was producing hit.
WWII-themed media was still popular at the time. And ST:TB was WWII in space.
War of the Worlds themes movies were popular (ID4 in 1996, and the War of the Worlds movies starring Tom Cruise that was released in 2005 – a mere four years before ST’09).
And you are discounting that Star Trek would not have had a major name attached to the move at some point. Again, Jendresen had eyes on Christopher Walken being involved in the movie.
The only thing known at that time was that both the audience and the studio was tired of Berman-era Trek; the audience did not like what was being produced, and the studio was tired of it not raking in money they thought it should have been making. It did not necessarily mean they were done with Star Trek in general.
They only needed Nimoy's involvement for the first film. Nobody even knew he had a cameo in the second movie. And the reason why the KT films have petered out is because of Paramount's utter failure with marketing them since STID.
Nimoy’s had a role in all three films regardless (arriving in the Kelvinverse and generally being the genesis of the story in ST’09, offering advice on dealing with Khan in ID, and posthumously with death of Spock Prime in BEY). I imagine if any future Kelvinverse films are made, Nimoy will play a part in some capacity, even if it just a tribute to him.
The writers of ST4, as far was we know, haven’t figured that bit out. But I have, as have some writers of the Kelvin comics.