Ultrawide support for loading screens and cinematics. Gameplay still only supports widescreen in "Windowed Fullscreen" mode as covered earlier in this thread. Even if you have a machine capable of taking the performance hit from running at a high resolution outside of actual Fullscreen mode, the game is broken in small ways that can be really distracting.
For example, see my attached image of what happens when you use a sight on a weapon. If you're playing a sniper (and I am), you see this a lot and it gets old. The sight is tiled on the far-left and -right sides of the screen. I've stopped playing until actual, complete widescreen support is added. It's ridiculous this still isn't fixed, but I'm hoping the updates to the loading screens and cinematics indicate they plan on adding complete widescreen support.
However, if tweaking the loading screens and cinematics so they aren't stretched (loading screens) or cropped (cinematics) is what Obsidian considers fixing widescreen, they're missing the point.