If they want to go by Omegascapes Canon and how it handled delta and sigma then it only makes sense for 5 bosses imo.

Sephiroth has to show up. The only enviorment they can honestly throw him in and it seeing relevant to why hes in FFXIV.

Only makes sense storywise to bring out the big bads of several games again, considering there are "other" remaining combatants of considerable power, then put Omega last.

09s - Kuja (IX)
O10s - Jecht/Sin/Yu Yevon/Yunalesca
011s - Ultimecia
012s - Sephiroth
013s - Omega

They could go three original bosses and omega too but that just doesnt fit the trend they had up till now. They could also use the 4 weapons from FF7 and then omega.

I still feel sephiroth is gonna show up. Theres really only him as the other villian that made such an impact on the fanbase as a whole besides kefka. To have one show up but not the other would suggest that Sepiroth wasnt considered as a contestant for this..