Looking at the map at the division between alliances I pondered the scope for more of Tamriel to be added as explorable in the future.
I noted that for the Aldmeri Dominion, the bulk of the Summerset Isles are not yet zoned. I believe pretty much the whole of Elsweyr hasn't been zoned either.
For the Ebonheart Pact the bulk of Black Marsh and Skyrim is still unexplored as well as the island of Vardenfell.
The Daggerfall Covenant seems the most problematic as regards scope for future expansion. Highrock is all zoned. There are bits of Hammerfell not zoned but not a particularly large amount left. Wrothgar is unaligned as are Claglorn, Hew's Bane and the Gold Coast, all in an area that might have ended up in the Covenant.
This situation may be moot depending if the long term expectation is to keep adding new zones to the world to add new content. Perhaps any new zones will be unaligned anyway so the lack of space for the Covenant to expand is not an issue.
Given the latest Hist expansion was just dungeons and didn't add a new Blackmarsh Argonian zone as I might have expected, maybe there might not be many if any new zones anyway.
My Characters
Xargothius: Breton - JOAT / Magsorc | Orchid the Fair: Orismer - Crafter / Heavy Tank | Voneri Vox: Dunmer - Magplar Healer | Rexorgiana: Imperial - Temptress / Magblade
Phirkius: Altmer - MagSorc | Rexorigus: Imperial - Med Tank / StamKnight | Sven Svedishchef: Nord - Provisioner / Stamplar | Finds-All-Fungi: Argonian - Alchemist/ Stamblade
Emerald-Wild-Guard: Bosmer - Wyrd / StamSorc | Nates Datum Festi: Imperial - Light Tank / StamKnight | Magnolia Desert-Blossom: Redguard - 2 Handed Stamplar | Shadow-Softpaw: Khajiit - StamBlade
Sorcerer | Dragon Knight | Templar | Night Blade