He could join the Imperials I guess. But two questlines? Not much.....
Exactly. I recently made another Altmer and just like my old one, he is a supporter of the Dominion and believes in the concept that Elven Supremecy will come to fruition in the form of an Elven-Centric Dominion replacing the Man-Centric Empire, just their destiny to rule. However, both are completely opposed to the Thalmor.
As for actually making a member of the Thalmor, you are not limited to Altmer, a Bosmer, or even a Khajiit can be one too. However you are correct that they don't really have anything to be a Thalmor. Pretty much a faction you can ally with or join.
Just saying, Altmer/Bosmer/Dominion Citizen =/= automatic Thalmor. I will say though, I love their outfits. Just a same that it's almost impossible to find an unenchant set of robes and hood.
This game was not meant for people to role-play a thalmor agent so it is going to be really hard to do so. even the college quests has you going against the Thalmor to some extent.
Well then you pretty much answered your own question. There's nothing for you to do. You'll never be truly Thalmor since they attack you on the road, you can't do the College because of Ancano, you can't really join either side in the civil war since the Thalmor first and foremost wants a stalemate.
Only even theoretical guild is the Thieves Guild IMO, if you think yourself as an agent establishing underground connections in Skyrim.
What I recommend you do is recreate the plot of a movie or video within Skyrim.
For instance, http://www.pcgamer.com/2013/08/04/mod-of-the-week-gifts-of-the-outsider-for-skyrim/ downloaded a mod that adds Void magic and proceeded to recreate the plot of Dishonored.
Thalmor assassins will randomly attack you no matter what i think. They are not thalmor patrols but a different random spawn with a note saying to kill you for interfering with the thalmor (even if you have not done anything of the kind yet,)
Price of being a secret double agent. Even your allies don't know your an ally, all for the sake of appearances