Okay, so personally, I hate fights that are over in 1 or 2 rounds. Even 3 rounds doesn't feel like enough player turns for me unless it's a pretty easy fight. However, I also don't want fights that necessarily take 6 hours of real time, or fights where there are so many enemies that the players are overwhelmed and it's too deadly. So how to make fights longer in ROUNDS, but not necessarily longer in real time, or deadlier? Well, obviously one solution is to use waves of enemies, reinforcements, or the like. But you can't do that over and over, it will seem repetitive and contrived. You can also use very large maps and require the PCs to move around them for objectives or the like, but again this is more of a "once in awhile" solution.
So here's my idea. What if you put as many enemies on the field as you want, but not all of them get to act each round. Instead, the enemy side only gets as many activations as the hero side does. Maybe you have 10 groups of enemies on the board, and 5 PCs, so each round only 5 of those enemy groups get a turn, the others hang back. This isn't even that hard to believe, if you've ever been in or watched a real world fight between multiple people. Not everyone is just constantly attacking with no break.
In this way you can have an enemy force that, if they all acted at once, would be way too deadly for your PCs, but instead they just rotate out which ones are active. Since all of those enemy groups aren't taking turns, it doesn't make the individual rounds take longer in real time. And it doesn't absolutely crush the PCs. But since there are more enemy groups to defeat, the number of rounds increases. Which means players get more turns. Which is something I'd like. I mean I wouldn't do this on EVERY encounter, sometimes you could just have encounters with a smaller number of enemy groups and they'd run as normal. But I like the idea of having this in the toolkit. Does anyone else think this would work?