I'm sure the devs already have a complete system in mind, but if their is room for any player feed back lets start it here.

this has been posted many times since beta but since BRs are to reworked here it is again:

Battle Regimens (BRs) are basically Skillchain from FFXI, lets take it a step further and borrow from other classic SE games.

Chrono Trigger: cross Tech skill system
+
Final Fantasy Crystal Chronicles: Magic stacking system.
+
FFXI: Skill chains

Allow mages and melee to enhance attacks with elemental magic during BR.

i.e. Thunder and Spin Stroke creates Thunder Stroke. (add unique animations)

Allow mages to stack attack magic, Flare and Tornado creates: Inferno

Allow support mages to combine magic for extra effects: Protect & shell = Barrier

Circle blade and Doom Spike = death spin (lol)

the combinations can be limitless, just use your imagination and leave it up to the player base to find those combos.

Or create quest content around unlocking "combo tech" skills so there is a more limited but very useful abilities for use to play with.

Lets hear ideas!