The main changes I would want are the modern interface/macro/addon system and some form of manual group forming tool. The vanilla interface was both clunky and allowed for things like macroing full rotations. Changing these to modern versions would count as fixing broken things. While an LFR/LFD type of automatic cross-realm grouping tool would be a deal-breaker for me, I would like to see a manual grouping system for finding group members from your own realm. A broken version of this did exist in vanilla in the form of meeting stones in front of dungeons. However, the tool should not be an interface button, but rather accessible via those meeting stones and possibly via major cities.