Except its not quite that complex. Building a spec for a new class is quite a bit different than building a spec for an existing class. Let's say I wanted to develop a class called Geomancer, and I wanted it to tank/dps/heal. They would be forced to wear mail armor, shields, and probably wield axes and maces. Right off the bat I'm running into problems because lore dictates that Geomancers are elemental warriors. There's already an elemental warrior in the game, and that is Shaman. Now I not only have to develop new spells, I have to develop new spells and mechanics that won't overlap with Shaman. I also need to figure out what stats this class is going to use, what type of range and melee this class is going to use, I'm going to have to develop an entirely new talent tree, etc.
Meanwhile, if I just create a Geomancer Shaman spec, the majority of those problems are gone because the theme of the class is firmly in place. The talents may need some tweaking to get the spec to fit properly (though some may need an overhaul), but in general, I won't need to come up with an entirely new system for this spec to operate in. I won't even need to come up with all new abilities. I can use abilities from other specs, and even use some old ones from old WoW that may have been discarded. Right off the back, if we're building a Shaman tank we can immediately bring back toughness and 2 handed specialization, or shield specialization, and anticipation. We can utilize Rockbiter weapon and maybe even Frostbrand Weapon right off the bat since they are barely used. We could take Primal Strike and make it into a powerful ability since Enhance barely uses it. We know that this new spec will probably share the elemental shields from the other specs since that is a class theme, so we can give this tank Lightning, Water, or even Earth Shield. Heck, we could even make a brand new shield. We have the 4 elemental shocks. We have Ghost Wolf for mobility. We have the totems for utility.
The skeleton for a 4th spec is in place, and we didn't even create any new abilities yet.
I don't even know where to start with the Geomancer class.....
If you're talking about PvP, PvP will always be unbalanced and broken. It will never be equal across the board.