Tactical combat in multiplayer is really my primary concern about the game because when there are more than two human players it may very well ruin the game to just sit and wait for other's combats to end, feeling helpless about how it goes and how long it lasts

Most games tend to make tactical combats last one turn only but I do think it could be better to have them last potentially over many turns. For instance, each tactical combat could have X turns (let's call those X turns a round) every game turn. Most simple combats would end in one turn, but those who can turn the tide of a war could last a while. This would very well allow more armies to join in as reinforcements, or even third party armies to come and wipe both weakened armies.

Why not give the choice to the attacker to withdraw from combat before or after each round (in order to prevent unwanted neverending battles when a simple raid was intended), making "raids" possible, while he could no longer withdraw if another armie joins in (he'd be trapped). A smaller army could then raid a bigger one a round to "weaken" it, or a big armie could have the combat last several rounds, time for reinforcements to arrive. 

In one game turn, there could be several rounds IF another army would join the battle in the same game turn : the first round would only involve the two players that initiated the battle, the second round would involve a third army (from one of the two or from a third player), etc.

In my opinion this could lift the combat system up to the "strategic" level instead of just "tactical", like it currently is in most games. I don't think it's reallistic to have two huge armies exterminate each other, with maybe reinforcements very close but not enough to be there at the very begining of the battle, which would have very well changed it. In most games, one good army can kill many other good armies by just eliminating them one after the other in the right order, having not to face the reinforcement problem. t's as if even the most longest battles were instant from the rest of the game's point of view, I think it's too bad.

Of course, the number of turns in a round could be set by players before the game is launched.

I'm not sure how hard it would be to implement such system instead of a normal tactical one, or if it would be moddable... well, just sayin' (sorry if my english isn't perfect and may make it hard to understand what I mean )