Take it back to early Wotlk type of gameplay design. Restore the systems, itemization, world, community, and everything else Blizzard removed from the game that were vital to its success. Remove all the gimmick systems, LFR, LFG, transmog, pet battles, 6000 different mounts, and all the other clutter that simply was just fluff to fill in for the lack of real design.

Replace the current development team. At some point WoW had a major change in the development process and a change in who was developing it. Both of which spelled a serious decline in WoW's popularity. The original development team had some real talent, but of course corporate politics (greed) eventually got in the way. I'm not 100% sure the blame can be solely placed on the current development team, in fact I know it cant due to corporate influences. However, I haven't seen really anything that's impressed me for many years.