Everyone loves explaining how they think Blizzard should fix everything. Blizzard never listens. This is why I play FFXIV instead. But...
Mercenary Mode is one of Blizzard's attempts to balance PVP. Added back in patch 6.2.2, it allows players of one faction's races to join battlegrounds as another faction. A race change (for the duration of the battle) comes along with this, including changes to racial abilities. But otherwise, it's still Alliance vs. Horde, the way it's always been, and probably always will be.
Well, great, but... at some point, it feels like you're just hanging onto this structure because factionalizing your player base causes problems, and you can't break that without breaking everything you've done. But how could you inch away from it?
In my proposed alternative system, every battleground has two sides that are chosen at random from a pool of new (or existing) NPCs. Blizzard is very, very good at crafting outsized personalities, so do that. Orcish clan warlords, human generals or knights, Tauren separatist factions, whatever. Bonechewer vs. Frost Wolf. Aldor vs. Scryer. Tusuhi vs. Huojin. Whatever.
When you join a battleground, you're split up evenly by side. If you expressed a preference, and there's a balance of people who did, you'll get the side you want, but it's not guaranteed. From there, battle proceeds as normal.
Why express a side preference? Each NPC will provide some unique reward, or class of reward, for fighting under their banner. A nice transmog set, pets, mounts, new hats, whatever shiny thing PVPers like to chase. Fight enough battles, or get into enough BGs where your chosen side appears, and you'll collect the whole set.
Ultimately, this not only solves the problem of battlefield population balance, and opens up new options for players, but it does something that is to me much more important: it leans into Warcraft's theme of war in a way that doesn't institutionalize and reinforce the inherent racism of the faction system they have now.