Umm
ok.

Games that has sequel/same number:
-FFVII: AC/DoC/BC/CC/TLO
-FFX: FFX-2
-FFXI: CoP...
-FFXII: RW
-FFXIII

Games of remakes:
FFI
FFII
FFIII
FFIV

futur remake:
FFV
FFVI

leaving...


The remaining games:
FFVIII
FFIX


So... FFVIII would be a great prequal/same time. Playing as Laguna & Seifer.
''not sequal''
unless there's a remake.