Does the sneaking ability ever improve with ranks in eso and if it does not will they add sneaking ability ranks in elder scrolls online to get better with sneaking
Everyone has the same, base ability to sneak. As in, range to stay hidden from enemies. The only way to augment this is through different skills lines, potions and some gear effects.
So while there aren't any sneaking 'ranks', there are ways to get better at sneaking without "leveling" it.
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Sneaking does not improve in rank, but there are talents in the Medium Armor skill line which improve your sneaking ability should you choose to go that route.
It's also possible that in a later patch they've talked about where they'll include the thieves guild questlines that there may be perks and such for it through that.
So does Stealth affect aggro radius for characters of all levels the same?
Yes. You'll need Improved Sneak from medium armor to lower the radius at which you're noticed (or the racial passive Bosmer and Khajit get).
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Not sure how well it works against other players myself but I imagine the same standards apply. Don't go where they can see you and you're fine. Not sure if third person view affects that though. It might just be only if they are facing the stealthed person.