I dont know if someone say it before, but we can "copy" FFXI skillchain system but easier.

We all already have 1 button for LB, so we keep all like now with a simple change when
Limit break bar fill up after 1º player use it you have a time window to follow up

And add a bonus DMG counting how many ppl follow, so 2 players "chain" 10% 3 players 20% 4 players 50% extra Dmg.

Its easy, adds a "TEAM SYNERGY" and all tank, heal and dps take part.

You can add diferent effects, maybe diferent chain based on who start the chain, im always think this is a easy way to make it better with a lot of depth to improve.