How much should a single character be allowed to accomplish in 1 turn?
What you’re looking for is a sweet spot.
On one end of the spectrum, you have: “That guy is able to do way too much before I get another turn.” (Munchkin Quest suffers from this, as described and fixed in my house rules.)
On the other end of the spectrum, you have: “I’m not able to do anything interesting this turn.” (Imagine a game of Monopoly in which you could either roll the dice to move or buy the property you landed on, but not both. The balance would not be appreciably altered, but I suspect the game would suffer nontheless.)
The reason games use interrupt actions — such as attacks of opportunity — is to widen the tolerance of the sweet spot: It allows you to design turns that are long/large enough to maximize the potential for “I can do something fun with my turn” while simultaneously preventing the “why can’t I do anything to stop him from that long sequence of actions?” problem.
The disadvantage of interrupt actions, however, is the complexity which arises from tracking the triggers which allow those interrupts to be used. (Attacks of Opportunity in 3rd Edition gave you a single interrupt action, but had a very lengthy list of possible triggers. Swift actions were later added to the game, giving you a plethora of interrupt actions to choose from; but these almost universally had the trigger of “whenever you want to do it”, which is very easy to keep track of. 4th Edition simplified the list of triggers for Attacks of Opportunity, but drastically increased the number of different interrupt actions in the game and gave most of them different triggering conditions — thus radically increasing the complexity of the game.)
THOUGHT EXPERIMENT: Imagine a 3E-style combat system in which every character gets a single standard action each turn. In addition, they have a single interrupt action which they can use at any time. (For example, to hit someone running past them or trying to run away from them.)
(If you wanted to keep full actions, you could allow them to be taken by any character using their interrupt action on their turn.)
The point is that you don’t define any trigger conditions for those interrupts: You don’t need to keep track of them (or try to avoid doing them). You simply use them whenever you want to and can do whatever you want with them. Theoretically, this would give you the flexibility to respond to actions on the battlefield as you choose, without the difficulty of trying to keep in mind all the possible trigger actions.
Certain effects could vary the number of interrupt actions a character has. For example, a haste spell might simply grant a character an extra interrupt action. (Although on a similar note, you’d probably want to prohibit spellcasting with an interrupt action. An added layer of complexity, but probably necessary for balance. There are almost certainly some other wrinkles to work through if you wanted to use a system like this.)
Sounds a little bit like MegaTraveller. If I remember correctly, characters could interrupt based on pretty much anything but I think only one interrupt could be active per side of a conflict to prevent an endless chain of interruptions. The play example I remember was – one character dashes from cover to try to move closer to the enemies. The GM has one of his gunmen NPCs fire at the dashing character. One of the other PCs pops out to offer suppressive fire. With one interrupt active per side, they get resolved in reverse order with the suppressive fire being resolved first, then the shot at the dashing PC, then the dash itself that started off this whole thing.
In a home-grown system I did once, I gave everyone an action and a half every round. They could do one thing at +1/2, or one thing at full bonus and another thing at half bonus.
That way, you could get some combat master taking 2 actions, but a combat novice could focus and get that +1/2 and still be in the running. It was interesting, as a way to get at the dilemma you describe.
I was mostly interested in cocking a gun then shooting, or riding and fighting, or interrogating and charming; various combined actions. It’s always an interesting balance.
I had an interesting idea myself a while ago very similar to this that I detailed here:
http://www.enworld.org/forum/blogs/herremann-the-wise/2035-dreams-5th-edition-how-initiative-should-work.html
It changed the idea of initiative into a semi-cyclical form and allowed characters to act when they chose to act as long as they were able. Looking back at it, it is still unrefined but I think there is the nucleus of a good idea there. Similar to what you ponder, the higher level your character is, the more “swift” actions they can perform. Think of it a little like combat reflexes but where the number of extra swift actions is determined on several factors rather than just dex modifier.
Best Regards
Herremann the Wise
Interrupts in 3e were an attempt to avoid saying no in the rules. The active player had to be allowed to do all sorts of stupid things in melee, but the players themselves had to be able to prevent the monsters doing the same thing.
But the game works better if you do say no. No, you can’t normally wander into melee range of an opponent and carry on walking. No, you can’t normally attack in melee and then walk away before they get a chance to hit you back. No, you can’t normally dig through your pack for potions while people are hitting you with swords. No, you can’t normally flip up off the ground and attack like nothing happened. No, you can’t normally charge up to something with great reach and attack with a shorter weapon. No, you can’t normally cast spells in a melee.
All the exceptions that prevent opportunity attacks in 3e can break all those negatives the same way.
You don’t even need overwatch type actions if you allow cover fire to leave a hazardous zone, or melee fighters to extend their sticky radius when not otherwise engaged.
Spellcasting can be disrupted by taking damage (or getting hit with a Dispell Magic) since their last action. More active type countermagic might use fire to negate ice if cast within the next round. Exceptions and saves as usual.
“THOUGHT EXPERIMENT: Imagine a 3E-style combat system in which every character gets a single standard action each turn. In addition, they have a single interrupt action which they can use at any time….”
Funny, this is almost exactly how my house rules do combat.
Everyone gets their full action on their turn, and everyone gets one interrupt to use whenever they want, no triggers. Interrupts cannot be interrupted (no strings of backwards-resolving actions.) The only instant cast spells are interrupting or defensive in nature.
I found this format necessary because I wanted everyone’s actions to be resolved by the end of their turn, particularly spellcasting. For that reason, no special benefit is conferred if you save your interrupt for your turn, because they are specifically intended to be used outside your turn. In fact, most of the interrupt abilities would be pointless to use on your turn.
The other side to this is, I don’t have attacks of opportunity, for a few reasons:
1) Since AoO’s are triggered only in melee range, ranged combatants are less likely to get one, making the rule favor melee.
2) AoO’s unnecessarily increase player’s DPR, especially in my rules, where the average at level 1 is 11. (You could say I have basically worked AoO damage into everyone’s normal damage.)
3) With the combat format above, an AoO is totally unnecessary for use as an interrupt, as there are several other ways to interrupt that do no damage.