In my opinion, and I think its the same opinion from many other people from the community and the discord server, your focus should be on all the current top 15 fighters before thinking about adding any legends or cross-promotional fighters.
Here is my list of MUST HAVE fighters and their current rank in the UFC:
Heavyweight:
8 Serghei Spivac
10 Marcin Tybura
13 Alexandr Romanov
14 Marcos Rogerio De Lima
Justin Tafa
Light Heavyweight:
10 Ryan Spann
Tyson Pedro
Ion Cutelaba
Middleweight:
7 Roman Dolidze
10 Brendan Allen
12 Nassourdine Imavov
14 Chris Curtis
Caio Borralho
Welterweight:
7 Geoff Neal
9 Sean Brady
Lightweight:
10 Benoît Saint Denis
11 Jalin Turner
15 Drew Dober
Featherweight:
9 Movsar Evloev
10 Bryce Mitchell
14 Lerone Murphy
Bantamweight:
11 Jonathan Martinez
12 Umar Nurmagomedov
13 Ricky Simon
14 Chris Gutierrez
15 Said Nurmagomedov
Flyweight:
6 Matheus Nicolau
8 Manel Kape
9 Matt Schnell
10 Muhammad Mokaev
12 Sumudaerji
13 Steve Erceg
These are the names that have been in the rankings for a long time and deserve to be in the game. And that's not even all of the currently missing top 15 fighters.
Again In my opinion, the top 15 guys in every division should be must! And I think you should put a special focus on the Heavyweight, Bantamweight and Flyweight, as there are really a lot of names missing here.