I'd like to run a war, but I'm not a huge fan of the PCs effectively disappearing while a single player conducts the war, or they stop being PCs and just start controlling armies.
I like the general design of the Mass Combat rules in Ultimate Campaign, and there is a small section concerning PCs in a battle, but it doesn't go into great detail about it.
Anyway, what I want to do is basically use the normal Mass Combat rules and have the PCs go running around the battlefield accomplishing tasks and fighting knee-deep in the war, Romance of the Three Kingdoms / Dynasty Warriors style. Basically, the PCs would be lieutenants, captains, or even generals of the army, but be directly involved with the fighting.
As it stands, what I'm thinking right now is that I'd segment off conflicts within the battle into key locations or Areas (since real warfare is rarely just two gigantic armies smacking into one another).
Whenever PCs enter an Area, they begin a Combat as normal - roll for Init, all that. The PCs then have complete control over their individual actions and the fight is just one giant melee with lots and lots of mooks.
Combat taking place in Areas that Party isn't currently in aren't in would operate on the Mass Combat Phase rules, as an abstraction of the battle happening where they don't have personal control of the situation.
The Ranged Phase would take a single Round when appropriate, and each Melee Phase would occur once every 10 Rounds, to create a time table of events.
PCs entering areas where the Melee Phase has already occurred will encounter troops who've already taken damage, thus the numbers on both sides of the conflict would have individuals removed appropriately.
So, that's the basic outline of the design - what I'm wondering now is, is this a decent way of dealing with the PC-level and Army-level encounters, and is the timeline too small (should the Melee Phase be spread further apart than just 10 Rounds)?
Also, what kinds of boons, bonuses, etc. should be incorporated into it, and how do you think events should be handled with in (such as destroying a bridge, opening a rampart, transporting an official, etc.)?
If you really want the Musou feeling, one suggestion I'd make is have regular troops, the run of the mill foot soldier types, be one hit point wonders. Basically, if they aren't important enough to merit a name, they fall like blades of grass before the might of your heroes.
You don't need to point this out to the players, they'll just feel badass scything through the ranks.
For named battles, try to keep it snappy, instead of fighting with an eye towards strategy, have named opponents utilize a specific set of attacks/actions. Where possible, try to meet the players where they are, which is to say melee fighter vs. melee fighter, ranged vs. ranged, so on. If you and your players are up to it, banter also helps establish the mood for a cinematic one on one combat.
I remember there being a rule somewhere in 2nd edition D&D called "Heroic Fray" or something to that effect that when in mass combat and the enemies were 3 or more HD less than the players they got double attacks and double damage or some such as that. It may have been in one of the Players Option books.
Dotting for interest, ideas, and suggestions after I get home.
Take a look at the mook rules from 13th Age: Mooks. I personally think that those rules duplicate the Dynasty Warriors style of combat pretty well.
What about things gained by capturing an area, etc?
Ultimate Campaign suggests +4 OM, but says other things might be gained. Aside from basic resources and maybe some typical loot, I'm trying to think of cool things to "drop" in those cases.
Maybe unique, one-time use items that negate a damaging aura of a specific type within a different area; or one that reveals/grants access to a new area where the enemy commander is hiding.
Also, consider introducing more groups that can field their own forces and have them as potential allies or enemies. A cabal of wizards that has forces comprised of various homunculi led by members of their order, or a large tribe of lizardfolk who just want to be left in peace (and get violent towards both sides when the conflict inevitably spills over to their swampy home). Will it add to the complexity? Yes. Is it worth it? Yes!
I see that someone has already suggested adapting the mook rules from 13th Age. Those make great rules for when your zoomed in and focusing on the PCs as active combatants in the fight rather than just commanders who issue orders. An addendum that I think warrants a mention: have the mooks able to absorb other allied groups of mooks with appropriate adjustments to HP. This can cause a sudden turnabout when the PCs think they can just mop up a lone group of enemy mooks only for a wave of mooks, with more than enough HP to be a serious threat to the players, to join the stragglers at the last moment and forcing a change in tactics.