How about adding new Merit Abilities that are powerful (not dumbed down versions) that can be used based on your available merit points. You could even go as far as allowing every job to use every other jobs 2hr but restricted to your current subjob.

Example:
BLM/RDM can use chainspell but the chainspell will drain 10 Merit Points.
NIN/WAR can use Mighty Strikes but will drain 10 Merit Points.
WHM/BLM can use Manafont for 10 Merit Points.

Could even allow other jobs abilities over the current /subjob cap which are merit abilities.

For example:
WAR/SAM can use shikikoyo for 3 Merit Points
WAR/SAM can use blade Bash for 2 Merit Points

Can change the point values around all you want. But it would allow people to make use of merit points once you get capped on everything you would ever need - would give people a reason to want to exp for even a little exp - wouldn't be overly overpowered since it would be dependent on your sub job and your speed at getting merits and their use would be limited as I said dependent on how fast you can get them. In abyssea this would be kinda fast depending on how good you are at keeping chains but outside abyssea it wouldn't be that common to get massive exp going.

Just tossing around ideas.