I'd love to see GREP be able to insert a character given a certain condition.
Imagine being able to set a GREP style within a paragraph style that could recognise a certain pattern and then insert brackets or maybe a return. A bit like a find/replace GREP I suppose, but within the paragraph style.
That is very unlikley to ever happen. Aside from huge overhead having to constantly monitor changes in your text as you work (as there is now with GREP styles, and more than a few can slow a long doc to a crawl on a marginal machine), there would need to be a complete reworking of the program architecture -- styles cannot add or remove text, only change the formmatting of what's there.