The thing is, The factions still need to exist to an extent. However, they do need to go away as a game mechanic.
Remove the faction barriers for PvE content. You can keep it for instanced PvP, and either keep them for World PvP, or turn it into a giant free-for-all. But, the big thing here is removing the “faction imbalance” that plagues PvE content, especially at high levels like Mythic raiding or 15+ and higher keys.
Also, allow cross-faction chat. It just makes no sense to have it restricted by “languages”, when everyone in the lore seems to speak perfect English anyway, despite several races having good reasons not to (The Kalimdor members of the Horde, the Night Elves, and the orcs and Draenei, for example).