Open World PvP What I voted for.
Simply put, this is the only real option for "Open World PvP." The other options are either not open, not world, or a combination. (I will hit them individually).
Open World PvP suggests PvP with unrestrained population, which can happen anywhere that PvP is enabled. This is what PvP and RP-PvP servers are for, otherwise people roll on PvE and RP servers. Some of the best PvP I have ever encountered has not only been Open World PvP, but Large Scale Open World PvP, with hundreds of players on both sides. It's much better in RP-PvP settings, but as that's not the topic I wont continue with it. Large Scale Open World PvP is dynamic, it's complex, and requires tactics and strategy to succeed. Open World in general allows the individuals to go out, pick a fight, and within an hour, spark a war with a hundred people on each side.
That is, of course, except on this game. The game world can't support more than a few dozen people in the same area without completely breaking down. Fights with a hundred people, a hundred fifty people, the game becomes a slideshow on the best of machines. Slightly lower than best machines, and it's a disconnect. If this problem is not adressed, then the only World PvP you are encouraging is that with lower numbers. This is often the "ganker" type PvP, which isin't fun, especially if you can't immediately call in a hundred people and lay waste to the nearest enemy settlement in retribution. Fix the game engine, allow the worlds to support a few hundred more people without completely failing, or improve the sharding system, and the Open World PvP will be some of the best PvP in the MMO community (as can be had since DAoC, anyway).
There is also the problem that on most worlds, the two factions almost never run into each other while questing. It's been mentioned repeatedly, but it needs to be. Most World PvP starts simply, from 2 questers killing each other and calling in their friends to help. It's spontanious, and its fun. When I was levelling my main, from 1-50, from early access, I encountered 3 enemy players. All on the same world, all within a day of each other. One was about 10 levels above me, and the other two were questing together, but the fact those two are the only random interactions is pathetic. On a PvP server on an MMO, you should be fighting your enemy for what you need, not in your own little questing zone where you need to go out of your way to find them.
What we have now is a poorly implemented Open World Warzone on every world, without faction limits and without PvP rewards. The sharding is a good idea for a solo game. But, as an MMO, it should never have even been a consideration. We know why it was done, and its understandable, but that doesn't make it right, or good; it makes it a quick fix.
Give rewards for Open World PvP. Tokens for killing people, bonuses for killing people higher level than you, no reward for killing people that are grayed to you, etc. That would be nice, and encourage people to fight elsewhere than in Warzones, which after awhile, I just want to hit my head into a desk over.
PvPvE balanced
This is basically what Ilum -should- be. Say, for every enemy player, 2 NPC guards spawn and the turrets attack a little stronger, and the points take a little longer to cap. That would fix the current major issue with Ilum. This is good for fortress, objective-based PvP zones where there are actual rewards involved and being outnumbered 10-1 is a distinct possibility (whereas in Open World, you can just leave and fight elsewhere, split up the enemy, etc).
But this is not Open World PvP. Open World suggests out, in the world, open and free, to PvP as we wish. That means, sure, sometimes we will be outnumbered 2 to 1, or 3 to 1, but I have seen fights like that where the underdog -wins-, and it feels good to win in a situation like that.
I would like to see this implemented for Ilum, and any other specifically "PvP" zones with objectives in the future. It would make it fun, and a real battle. If theres a hundred of one side and 10 of the other side, they wont be completely smashed every time. The enemy would actually have to -work- a bit to win. There's plenty of opprotunity with this option.
Faction Population Cap
This is a Warzone, simply without a goal or rewards. This is -not- Open. This can be a good temporary fix for the sharding system until the game engine is fixed to allow more people to join on a single shard, but generally speaking, no. Maybe for zones like Ilum, -maybe-. But if I want to play strictly even numbers, I'll play warzones. I dont need Open World PvP to be as regulated as them too, thanks.
Guild Based
Fun for sandbox games, not these types of games, especially with this setting. This will almost kill RP in the game because, quite frankly, seeing organizations fighting each other on both sides makes no sense. Maybe on a specific server for this type of PvP, it could be fun, but in that case, as others have said, mechanics will need to be in place to make smaller guilds as competative as larger ones. Bonuses, alliances, etc.