'Raw' Open World - faction vs faction, with no faction population restriction mechanics AKA 'true' Open World PvP. Factions claim objectives.
Yuck. I'd think your own experiences with grotesquely mis-matched faction sizes will do more to put this (wretched) option off the table than any poll, but I'm still here to say it out loud: the only thing worse that advancement by RNG, is advancement by population imbalance.
PvPvE balanced - bolstering the underdog faction through NPCs, turrets, etc. Factions claim objectives.
Played a fair bit of this style in LotRO, and on the whole its pretty sad and annoying. 3 guys hunkered under a pack of friendly elites while 39 enemies plink at them but refuse to commit to any sort of real attack does not exciting gameplay make. I like intereactive environment bits, but as a balancing mechanism for advancement by population imbalance... it still stinks.
Faction population capped - strict balancing in place between faction populations in objective areas. Factions claim objectives.
I'm unfamiliar with previous examples of this, but I'd be willing to try it out. In much the same fashion that warzones brought me to advance two characters to battlemaster when I rarely PvP at all in other MMOs. I don't mind having 4 people pile on me when I know that it's freed up 3 of my teammates to get the job done elsewhere.
Guild based - everyone is your enemy except players in your guild. Guilds claim objectives.
I despise anything that gives direct advancement advantage to mega-guilds. Advancement by population balance by a different name. I preffer to opperate in guilds smaller than 20: these are people I know, people I go over and have dinner with in person once in awhile. This means I shouldn't play in the new sandbox at all? Kay...