when someone plays new vegas it should be absolutely clear that it contains some very important core rpg mechanics that fallout3 doesn't come close to having.
Perhaps. I agree, Obsidian does certain things (like quests & quest structure) better. But they do "world" worse.
I play different games for different things. I don't expect the same from a Beth game, or a Bioware game, or a JRPG. I know that they have different strengths, weaknesses, and focuses. I know this going into them, so I don't get disappointed when I play, say, a Beth game and don't get Bioware-style character interactions. And I'm fine with different games being different - I think the whole "I wish X's games were more like Y, because I like game mechanic Z" is a stupid argument - I wouldn't want everything to be the same, variety's a nice thing to have.
And like I said - maybe that's part of why I was disappointed by FO:NV. Incorrect expectations - I went into it thinking that it would be Beth/FO3-style open world exploration. Instead I got something different.... Obsidian's semi-linear, complex-quest-web, faction thing. Set in a much less open and exploration-friendly world that just happened to use the same engine as FO3. So yeah, some of it is the fault of having false expectations. But I still don't enjoy the game, at a basic level, as much as FO3. And it still wasn't a game "in the Beth style".

I expect Obsidian doing a TES game would end up kind of like Oblivion running one of those "level via XP" mods.... kind of a generic fantasy RPG that has the trappings of another series. (i.e, Obsidian could make a fantasy RPG with Beth's "level by using skills" mechanic.... but it could be any random RPG, it wouldn't be a Beth/TES game. So why not make up their own original setting?)