I'm not saying that ESO should or is going to go f2p, but we all have to understand that this issue may come up down the road after release. I do not wish to discuss payment models of ESO in this thread.
Create from what's created
What I suggest is instead of converting ESO to f2p if it doesn't meet financial expectations, to make a game separate from ESO. This is very similar to what Bioware/Mythic did with Warhammer in Wrath of Heroes. Although WoH failed, moreso from design and balance issues, I think they had a great idea.
Bioware/Mythic used resources from Warhammer to make Wraith of Heroes. Development cost versus worth still may be an issue in an ESO conversion, that goes without saying, but it's not as costly as creating a new game from scratch nor will it jeopardize the integrity of ESO by converting to f2p after release.
What type of game?
I imagine this standalone game to be something like a battle arena. Players can purchase premade named characters (this model is used in many MOBAs including LoL and Smite) for a small fee as well as different skins/armor sets. They should also be able to earn currency to unlock these characters by playing (again popular in many mobas).
These playable characters could be released monthly, closely representing classes/playstyles within ESO. Appearances such as weapons and armor would be cosmetic for fair and balanced gameplay. The game modes would be variations of plots, ctf, deathmatch, or other creative objective-based matches... including 3 sided battles.
Again Wraith of Heroes was awesome fun in the first few betas. They introduced 6v6v6 battles and it was great. Where the game failed was in the pricing model and balancing. Players were greatly rewarded for doing well, like instagibbing all enemies if they had a long killing streak, leaving solo queues at a disadvantage. They also released elite tactics that was up to 3x better than other tactics but you couldn't purchase them outright, you had to buy a chest and then got a random elite tactic.
Entice the free2players to sub
You could also tie this into ESO and encourage subs by allowing subscribers the option to play with their character model. Zenimax could do this a few different ways; all skills in ESO could have their own version in this standalone game allowing players to play their character closely to how they play in ESO or they could just use the skills/abilities of a premade character within their class (but with their own character model).
DISCLAIMER: I think they should be careful not to reward players for playing the standalone game but rather reward subscribers for playing ESO. They should not tie these titles together so closely that people are discouraged from playing in Cyrodiil. I don't want to see them make the same mistake as Warhammer by draining their RvR lakes to fill scenario queues.
Why?
My first reason why is that I personally enjoy the MOBA genre and where it is going. The genre is expanding and redefining itself as time goes by. So far only Smite and Archblade use a different view other than iso/overhead. The popularity of battle arenas is growing, and it would be nice to have a dog in the race when Blizzard's Heroes of the Storm is released.
Second, it would be beneficial to introduce frugal players to ESO without having to pay first. The standalone game would function similar to a free trial using abilities and common playstyles in the game to show off combat in ESO.
Lastly it could add a way for players to experience instant battles without taking too much away from ESO and Cyrodiil. The standalone game would attract a different market and allow subscribers to also participate while keeping things separate from ESO ~ as long as subscribers are not rewarded in ESO by playing the standalone game.
Summary: Create a separate standalone game using the resources of ESO to appeal to a different market instead of going f2p (if it comes to that).