I'd like to see a variable length combat round, ranging roughly from about 1 second per round to 1 minute per round. That covers everything from GURPS and other such system emulation all the way back to early D&D emulation. That might prove to be a bit too ambitious. But even so, aiming for it should at least let us hit something close to a 3 second to 30 second range. It is not necessary that every possible interval in between be usable, though we do need at a few in the middle.
Don't Panic! I need to show the guts of this in seconds to explain it, but I'll try to show how it would be cleaned up at the end.
In order for this to happen, all things that happen in combat either need rules to scale to the different acceptable times, or readily fit into DM adjudication when they don't quite fit. I'm also embedding some assumptions about changes to magic here, that do have effects, but I think those effects are generally good for D&D--allowing major spells, but scaling them in the action economy differently than other abilities. I'll stick to 3 second intervals, for ease of multiplication, though I can see a case for 5 or 6 seconds as the common base, too.
By default, melee actions are on the smallest scale, movement and ranged physical actions are on a moderate scale, and magic is towards the upper end. We can assume that a system designed with this mind, however, will have exceptions. Some common examples:
3 Seconds - Swing a melee weapon once while shifting a 5' step.
3 Seconds - Throw a weapon, considered part of "melee" for these rules.
3 Seconds - Parry, Block, or Dodge a blow aimed at you.
3 Seconds - Pick something up.
6 Seconds - Load, aim, and fire a bow, while shifting a 5' step.
6 Seconds - Move your "speed" -- a relatively small number.
6 Seconds - Dive for cover.
12 Seconds - Prepare and cast a modest spell on yourself.
30 seconds - Prepare and cast one of those "big gun" spells that D&D is so famous for.
Times are approximate here, and we'll do some judicious rounding as we go.
Of course, we aren't going to pull the AD&D psionics trick and have multiple time frames going on at once. Instead, we'll scale up or down the actions to fit the circumstances, and assume some of the same abstractions that early D&D used:
1 minute rounds - You can move up to 5 times your speed over the course of your actions. If you don't use this part, you lose it. In addition, you can cast one major spell, two modest spells, attack with a bow up to 3 times, or attack with a melee weapon up to 4 times. The attacks aren't straight multiples, because you aren't having to work for positioning here. Furthermore, the blocks, parries, etc. are subsumed into this. This is the most abstract version. A melee character can divide those attacks up among any number of targets that he can reach, including some that may be charging him, but must announce all targets before rolling any dice. Similar rules apply to the ranged character. Shooting into melee has some drawbacks.
30 second rounds - Still fairly abstract, defensive moves are subsumed into movement, but you are limited to 2 times your speed over the course of your actions. You can't cast a major spell unless you stand stock still, doing nothing else. You can cast one modest spell, and still move. Archers get 2 shots, and melee characters get 3 attacks, both to go along with movement.
15 second rounds - The halfway point between 3E/4E and early D&D, this is the last point before we hit more precise accounting by actions. You can move and do one thing, except melee guys still get 2 attacks. Archers can get two attacks by staying still, and likewise for casting a modest spell. Casting a major spell takes 2 such rounds, and you an barely twitch during (though remember defenses are still abstract).
In any of the above, you can switch weapons if you want, but you can't do that and use more than one type in the round. This is still fairly generous by early D&D standards, for the 1 minute, and by the time you hit the 15 second round, you probably woudn't have time to use one, switch, and use the second, anyway.
6 second rounds - A "standard" action is 6 seconds, and this plays a lot like such actions in 3E/4E. About the only differences are that melee guys can get 2 attacks by not moving more than 5' each time, while spells take multiple rounds. However, this is also where you'd put the "optional" rules to turn half of that 6 seconds into a "move" and "minor" action combo, and not allow the multiple attacks, for the 4E style. Or something similar, but allow the "full round" attack (still limited to 2, but no penalty), as an approximation of the 3E style. You could also have such a module to allow melee guys to use "attacks" to parry, which then forces ranged and spellcasting characters to delay when they need to dodge.
3 second round - This goes full intricate, where nothing happens unless you do it on the minor scale. You can activate defenses all you want, but each one delays whatever you were doing before by 3 seconds--that is, one round. This is practically slow motion. Arrows are nocked/drawn separately from firing. Spells are prepared for several rounds before they are released.
1 second round - I'm not even sure this is doable, and have left it out in order to keep this from being any longer than it already is.
Note, of course, that there will need to be some magic that can work a bit faster, if only as an option. It should be correspondingly weaker. Also, this presumes that damage from melee and ranged attacks is set to scale appropriately with this system, though it is not assumed to be exact. Each system does give some favor to different characters.
It's entirely possible that something like 3/10/30, or 6/20/60 second rounds, having only those three, would cover most of the useful scaling.
Finally, there is no round-to-round tracking built into this, except for casting spells at the more intricate end, and nocking arrows at the most intricate. Once someone got used to the system, if you absolutely had to swap rounds to a faster version after some intricate work, it would be easy enough to let the archer or caster get some free credit in the next, longer round. Going the other way is even easier.
