I would not want anyone but Obsidian doing this. I feel since they have worked with Bethesda that would give them a plus with familiarity.
Obsidian is a team who always stands their ground on their beliefs and is one of the smaller gaming companies still around, so they are doing something right.
I believe they would bring a uniqueness to the characters and game play. I would rather travel with Khelgar in NWN2 than Oghren in Dragon Age. I also would rather do the hacking in Alpha Protocol than the planet scanning in Mass Effect2. These are just a couple examples to their many creative differences.
Many will bring up bugs in their games but everyone has these. I was very impressed at how well Dungeon Seige played, especially co op. The menus were great and I have liked what they have accomplished in the dialogue wheel. Story has always been a plus for Obsidian and they have been very consistent with this.
Looking at the environments in their games I 100% believe they could handle a beautiful uniqe open world.
They would need the time to do this which I think this is where Bethesda would treat Obsidian with the greatest respect.
I love Obsidian and would wholeheartedly support this.
I would have a hard time with Bethesda handing their baby over at all,or to just anyone if this happened,but Obsidian would be my choice.