I want Obsidian to get another stab. I could see them pulling off something great with an improved engine and no sabotage like they got when they developed New Vegas.
The last time they let another studio develop something, it blew expectations out of the water and as a result made their game built on their home engine look like hot dog water.
New vegas is by far the beat fallout game, if for no other reason, it's an actual role playing game, and not an action adventure game.
To some extent they likely do. Nobody truly knows their "proprietary engine" other than dedicated modders and bethesda staff.
There's definitely a level of negotiation that goes on between Microsoft and bethesda, which, outside of their massive titles Skyrim and Fallout, has successful games published (not developed) by Bethesda, like Doom, Deathloop, Dishonored, among others. If Microsoft makes demands, they could backstab the devs of whatever game they make, just like they did to new vegas.