40 plates is the next logical step and would give some futureproofing if we are sparse and only want 1 plate per job, for every job.

21 Regular Combat Jobs
2 Limited Combat Jobs: BLU + Beastmaster (confirmed)
8 Disciples of Hand
3 Disciples of Land
= 34 Jobs total in Dawntrail.

If they continue at the pace of +2 Jobs per expansion (discounting potential Limited Jobs) this would give them ~6 years of futureproofing of additional jobs being released without running into any glamour plate issues whatsoever.

Doubling the glamour plates seems the most expectable thing they might do and they REALLY need to do this in my opinion - I am already trying to find "cross-over styles" between specific job types (like a dark knight glam that can be used for paladin as a dark paladin for example) to conserve glamour plate space as is.