They should have made Draeneis and/or Naga an actual playable race. It wouldn't be that hard to create the missing buildings, either, if someone made a proper melee build for them.
It would have been so GOD DAMN INCREDIBLE if Blizzard would have fixed some issues of the Editor and the systems. This way they could remake the original campaign to be better and with new stuff as well! This would have been already enough to sell use the Reforged.
Some Examples:
1. Quality of life improvements should be the priority.
1.1 Better pathing system. The way units collide with each other is unacceptable in this day and age! Starcraft 2 has a great pathing system. Blizzard should be able to do it and doing it good.
1.2 Custom keybindings in the Option Menu in-game. There is no discussion!
1.3 The UI takes to much space and can be handled much better but should have some resemblance to the classic UI.
2. The Editor is the best selling point for old time fans to buy the updated game.
2.1 RETURNING CUSTOM CAMPAIGNS!
2.2 Why not adding the option to increase the number of playable races/factions to make it more simple to select the race/factions the player wants to play?
2.2+ Changing Humans to Alliance, Orcs to Hord, Undead to Scourge. -> (This is a nitpick and not a must)
2.3 Increasing the options of the Unit Editor. Adding new functions to old abilities, new stats for units like the amount of units of this type you are allowed to train, maximum individual hero level, accessibility and so on. -> (can be really handy if you create a unit like the headhunter that can be upgraded to a different unit)