So the vanilla mob cap is basically 70 mobs per player, but if you add new spawns to potentialspawns.json and don't specify a maxcount, then they just keep spawning, ignoring the vanilla cap, until the game crashes.

While a static cap can be set in potentialspawns.json, or spawns.json, this doesn't adapt to changing player numbers like the vanilla cap does. The ideal situation is that the spawns added by potentialspawns.json would respect the vanilla cap. Another feature beyond this might be the ability to set dynamic maxcount's with respect to player count.