One particularly recurring change to fractions that arises when typing long texts is that from inline to display mode. It thus seems reasonable to have a variant for this, via TAB, since casting a fraction in one style into the other means a lot of cutting and pasting.
I know an inline fraction has no structure, but that's a very desirable thing to fix.
If this proposal is not implementable on the short term, "Delete structure" (alt+backspace) should leave both numerator and denominator as plain text, and the cursor between. That would greatly reduce the number of keystrokes for the casting.