Why not a simpler approach that the FFIV remake took- You could merit maybe 1-3 abilities from any level 99 job to be used by any other level 99 job. To prevent it from being ridiculously overpowered, you could go with your original suggestion of capping it at level 25 or w/e.

I don't think that would be game breaking, and it would be really convenient to have provoke on all jobs, or even access to drain samba 1 all the time or Light Arts / Addendum White (with only up to level 24 spells).