FE has an interesting system. I believe all units gain initiative at the rate listed into a pool, and the first unit that is >= 100 takes its turn. Upon taking the turn, 100 initiative is subtracted from that units initiative pool. This seems to eb the way things work after I looked at the Flurry ability from the Master's Affliction mod. The MA Flurry ability gives a +100 initiative self buff that lasts for 4 turns, causing the unit to fill its initiative pool instantly after acting three times in a row.
This creates a system similar to one you might see in an active time battle system rpg, such as Chrono Trigger, but with a tactical grid. Using Chrono Trigger as an example, one could wait in real time until every member was ready to perform dual/triple techs, or just use abilities in a different order than the characters' speeds.
So what we need is a command that lets the unit wait a certain amount of initiative, since initiative is basically time for all intents and purposes. The real question here is, should the waiting unit still gain initiative while waiting? If it were to continue gaining initiative, you could let a unit store up turns by waiting until the unit gained 200, 300, 400 initiative, then unleash those actions all at once. This would invalidate casting times. On the other hand you could just let the unit sit at 100 initiative while another unit accumulates initiative to 100. After the second unit takes its turn, have the waiting unit's turn come up again. I'm in favor of that more since letting a unit wait incurs a time cost, and prevents the loophole explained above.
To be honest, I would really love for another expansion to FE that adds a height factor to tactical maps, as well as some more mechanics like FF Tactics, where init facing matters. Basically just pull ideas from FF Tactics, since that game's tactical combat is superior to this one. Not that that's a bad thing since some people like it that way. I'd just like my games of FE to have a deeper tactical game, and I think making it deeper will allow the rpg side of things to become deeper, and by extension, the empire building and rest of the game.
Also, initiative should really be renamed to speed, to avoid the D&D confusion some players have brought up.