I'd support combos as long as it's more interesting than just increasing damage with each subsequent spell. 100 POT > 150 POT > 200 POT is fine as a baseline, but there needs to be something more.

Instead, I'd like to see either healing or supporting effects on the third spell in a combo. 1,2,3 for raidwide hot, 1,2,4 to decrease hostile target's damage for a few seconds, things like that. Maybe one for a long (30, 45, or 60 sec) dot, but definitely no direct damage combo that would be spammed endlessly.

Button bloat could be kept low if the order of GCD spells determines the effect. Think Mudras, but GCD and automatically triggers when the third spell is cast. Three spells could give up to six possible effects: 1,2,3 / 1,3,2 / 2,1,3 / 2,3,1 / 3,1,2 / 3,2,1.

Off the top of my head, some possible effects include a party hot, party direct heal, party shield, reduce damage taken by party, damage down on enemy target, increase damage taken by enemy target, party haste, aoe stun centered on target, decreased MP cost for party, create persistent zone at either self or target location (could either buff party in zone or harm enemies in zone), or increased healing by self or party.

If we add some sort of permanent buff can be given to a chosen ally (similar to Dance Partner) then there could be some single target buffs that apply to that player. I'm thinking of something to heal or support the MT for this.

Completing a combo could also reduce the CD on certain abilities, or replenish class resources, or build charges for an entirely new resource. SCH knocks five secs off Aetherflow by doing a combo, or AST gets a stacking buff from each combo and can consume three to draw a card, or WHM gets a stacking resource that gives +10% potency to the next GCD heal, up to +50%.