I can't seem to find an answer to this anywhere in the good ol' Googs but is it possible to explore all zones or are you 100% restricted to your faction zones + Cyrodiil?
Yep. One of the endgame perks is to be able to cross factions.
Living vicariously through my characters.
My Girls:
"If you were trapped in your house for, say, a year, how would you pass the time?"
Nephikah the Houseless, dunmer assassin: "I suppose I could use the break. I have a lot of business holdings now that need management." Swum-Many-Waters, elderly argonian healer: "I think that I would enjoy writing a memoir." Silh'ki, khajiit warrior-chef: "Would this one be able to go outside, to the nearby river? It's hard to fish without water!" Peregrine Huntress, bosmer hunter: "Who is forcing me to stay inside, and where can I find them?" Lorenyawe, altmer mechanist: "And why would I want to go outside in the first place? Too much to be done in the workshop." Lorelai Magpie, breton master thief: "I'd go nuts. Lucky for me, I have a little experience sneaking out!" Rasheda the Burning Heart, redguard knight: "I would continue my training to keep my skills sharp." Hex-Eye Azabi, khajiit daedric priestess: "I suppose it would be lucky, then, that I built a shrine to Mephala in my backyard." Yngva Stormhammer, nord bandit (reformed...ish): "I hate being inside even when I'm not forced to be. GET. ME. OUT." Madam Argentia, vampire dunmer aristocrat: "I suppose it would be more of the same. I have a rather... contentious relationship with the sun." Mazie gra-Bolga, orc scout: "Uh... I'd have to house train my bear..." Felicia the Wanderer, imperial witch-for-hire: "What Lorelai said." Calico Jaka-dra, retired khajiit pirate: "This one would like a rest from her grand adventures. Her jewel shop runs out of stock!" Shimmerbeam, blind altmer psijic: "Provided that I am confined to Artaeum, I do not think I will want for things to occupy my time." Shauna Blackfire, redguard necromancer: "Sounds like paradise. I hate people." Kirniel the Undying, cursed bosmer warrior: "I would feel useless, not being able to fight." Echoes-from-Dragons, argonian who thinks she's a dragon: "All the better to count my hoard!"
Wouldn't it be interesting to do a traitor quest line to change allegiances? I know for those of us who pre-ordered we can make any race any faction but make this option a mid-way to late level optional quest just to change up the dynamics and give those who RP another tool to really flesh out their characters.
Basically consider it 'new game+'. Once you finish the main story you have the option of playing the content in the other factions from their start, but scaled to level 50 (you won't see any of that faction's players).
So yea, you get to see the entirety of Tamriel as it exists in this game on one character.