Borrowing from a bunch of your ideas, how about this:

Players (humans) are officers in the armed forces of the sides/nations they represent and are issued mobilization orders according to the sway of the war and other abstracted issues. Depending on the nature of the war and the opposing parties, players may be mobilized from "home" (short duration missions) or may be camped in the battle region (long-haul missions). Rather than replacements being purchased, casualties are (eventually) replaced by the government/commanding officers, with an efficiency and effectiveness proportionate to the resources of the side (so world powers would replace units more quickly while guerillas much more slowly).

Players who disconnected for long periods without informing the game of intended absence would lose "preference points" (it would be construed as a lack of "readiness"), which would affect how quickly their requests for reinforcements/supplied/etc would be processed (priority queued).

This game isn''t necessarily balanced, though certain overwhelming elements would be left out of certain (most) conflicts (no nuclear missiles or other weapons of gross destruction). Geography would play an important part such that players would seek to use the surroundings to their advantage: a low-mobility guerilla squadron might try to move the conflict to swamp/marsh if up against a highly mechanized world power team, set traps and otherwise interfere with equipment advantage. world power teams would attempt to use superior technology to interrupt communication, divert supplies and significantly weaken the opposition, then use swift, powerful strikes to end the conflict.

Finally, players could - and this is only a possibility, but a likely one were this game ever implemented - be given the option to specify whether they wished to always play world powers/highly advanced teams, always play underdog teams or play any combination. The game''s political balance of power could then shift continuously and completely independently of the players, causing formerly powerful teams to get the experience of being the underdog and vice versa. The game could also be scenario based, though that''s a lot closer to a protracted team-based RTS than an MMO (lower persistence).

Just a couple of random thoughts.