...cosmetic mods for equipment (armor&weapons) mainly...
(wet dreams:-))

is there any chance for this TES tradition to continue?

It's more likely than other MMOs that they would allow cosmetic mods, but it's still uncertain. Probably UI mods, and those are likely to have some extensive capabilities.

I'm still hoping for an offline singleplayer or LAN-multiplayer version. Unlikely, but it would be wonderful.

Nope its an MMO. That means the only type of mods we'll see are UI based. Changing the look or amount of action bars, changing what the mini-map looks like etc etc, having information like combat logs and Boss ability's displayed. You'll never be able to mod textures and environmental details. It being an MMO any changes to the game files is strictly prohibited.