Reflecting on FFXI's Skill chains/Magic Bursts and FFXIV's current Battle Regime system, I feel that the current regime system is inadequate, confusing, and slow, but that its core goals are good.
The goal of XI's systems and XIV's systems are to cause players to work together when executing actions instead of just performing abilities that they want. The concept is "if I work together with my teammates, we can do extra damage." However, the current system has many problems, all of which have been discussed at great length in other posts. Therefore, I propose a new system, which is tentatively titled the "Unity" system, which contains Unity Actions and Chain actions.
The system would have 2 distinct type of actions. Actions where 2 or more players (based on the unity action chosen) both choose the same action to perform, and when they both have adequate stamina, the action is performed. Ex: A gladiator selects the "Magic Blade" Unity Action, while a Conjurer selects "Fire Blade." Once they both have enough stamina, the action is performed, in which conjurer casts a spell, fire appears on the Gladiator's blade, and he attacks for increased fire damage. This attack's strength would be more than just using similar stamina cost attacks.
When Player one uses his first action, an icon will appear above his head, and it will be indicated in the chat log that he is using a Unity Action. This way other players know someone is trying to do a Unity action without having to spam in chat.
The second type of attacks would be similar to skill chains/battle regimes, called Chain Actions.. I choose the appropriate chain action , and perform it. Within a time period another person must select another appropriate unity action, and so on, until a max of each party member performing an action on the mob. Once the actions are all performed, the chain is closed either via a single party member using a closing move, or the entire party using the same closing move. The closing move deals damage/heals based on the number of actions performed, and applies debuff(s)/buff(s) based on the actions performed.
Ex: Party member one uses a chain Fire spell, party member two then uses a chain lancer attack. Then they both use "Slow Finish" to end the chain. This attack deals damage based on quantity of moves, and slows the target.
You could have chain finishes that deal damage based on number of debuffs on target, one that adds debuffs based on number of different elemental attacks done, one based on simply number of attacks in the chain. etc. etc.
The goal here is to encourage people to work together, but not require any huge time investment during combat. The amount of coordination is no worse than a skill chain used to be, and while the Unity chain finishes would require the most coordination, since you have to do skills in an order, and then all do the same move, these attacks would be the strongest to compensate.
To sum up the actual abilities: Unity Actions: Both chars perform an action together. Chain actions: Party performs various actions in sequence and then finishes chain together.
Next, some system things: A PC builds up Unity points when he performs actions on a target while in a party. Unity points decrease overtime when one is in passive mode. The amount of unity points per action is based on the stamina cost. A simple attack may be worth 100 points, while casting water may be worth 200.
Next: Above the skill bar would be a unity bar on which one would place unity attacks. Unity attacks would be set ahead of time and cost points to set, like other actions. Setting a unity action costs UP not regular AP. So one could set "Magic Blade" at UP of 2, and then Slow finish for 3 UP.
Finally: the goal is for many of these attacks to be relatively cheap. So "Spell Blade" may cost only 500 unity points to activate, so about 5 regular attacks. This would make combat more more dynamic in a party.