I suggested a somewhat similar thing a while ago...but was having jobs equipable at any class it can use actions if it has the right weapon.

Example:

WAR can equip MRD, GLA and PLG actions...So if you have your MRD with 2-handed axe, GLA with 2-handed sword or PGL with hooks you can equip WAR on that class (it would keep the base class actions plus the WAR and crossclass actions).

PLD can equip MRD, GLA and CNJ actions so a MRD with 1-handed axe, a GLA with 1-handed sword or a CNJ with 1-handed club being able to equip PLD on that class.

This would add way more flexibility to classes AND jobs, allowing them to really become role specific. Who never thought WAR would rocks with Fists of Earth or that PLD could work welll with curaga and CNJ elemental nukes?

Of course my suggestion to them was way more detailed than this, but this is enough for you guys to understand the idea.

Pretty much would be an 1-handed axe that requires MRD PLD instead of MRD WAR, hooks that are PGL WAR, poles that are LNC MNK, etc. This way jobs could really be balanced in a way of becoming a role on top of a class. If the class equip PLD it is expected for it to tank...if it equip WHM it is expect to heal, etc.