More control over our passive systems in our body like metabolism and other things. Like thinking "ok, I ate a lot of calories for lunch, maybe I'll just not digest all that properly." And pain too, so like if you know somethings going o hurt but it's good for you, like getting an injection, you just decide not to feel it, because that pain isn't doing you any good. Not just in a mind over matter way but literally just not finding it unpleasant. And being able to focus on the work that's due for tomorrow rather than being drawn back to posting on the escapist. Just some sort of Mental Override system.
I mean it's pretty lame but also pretty fundamental things that it's surprising that we can't control. I know these could get badly abused and don't really advance us much but overall I think it'd be a cool thing to be able to do.