We have lovely typeahead search suggestions for regular page names, and for raw Special: page names (eg if you "special:rec" and get shown "Special:RecentChanges" and "Special:RecentChangesLinked") but...
What would come in quite handy for us power users would be if for special pages which are known to accept a page title as a slash-parameter, we could get typeahead suggestions for page titles!
Eg, type "Special:Whatlinkshere/leb" and see:
Special:WhatLinksHere/Lebanese Civil War
This would require special pages registering themselves as accepting titles as a parameter, as not all of them do. (And some take usernames or other special cases, which would need different support.)