I'd stick to having all the unit's original abilities be present in a direct capacity where possible and sensible for the map design. The mini-Earthquake Totem Pound is a nice runaround for Pulvarize's chance-to-cleave, though I feel Tribal Ancestors fits the Spirit Walker better, both due to synergizing with Spirit Link and due to the Tauren being the "dumb muscle". This does force some serious issues with the existing spellcasters, as they already have three abilities, leaving little room to make them self-sufficient without considerable work in enemy composition and twisting details to make the toolkit work well. Or having a defensive army the players need to turn the tides for to allow the support casters to shine. Or one could embrace the lack of self sufficiency, structuring the map around teamwork in much the same avenues as unit compositions.
My own thought for the Meat Wagon:
Corpse Stockpile: Merging the cargo and Exhume Corpses, so there's three variables in corpse type, corpse rate, and how much cargo space there is to store them, using this to give a full progression for the map's standard.
Blighted Mass: Inferno-based, makes a temporary but strong mass of flies that have on-hit and self-affecting aura DoTs, with increased levels improving the impact damage and duration rather than the quality of summon.
Carrion Feeders: A far weaker but permanent version of the Blighted Mass summon, improving the cost, cooldown, maximum and number spawned at a time rather than ever improving the quality of the individual summon.
Carving Tools: Engineering Upgrade that makes the corpses generated smaller and come faster, makes Blighted Mass more spammable, makes Carrion Feeder spawn better units, and gives a considerable speed bonus.
The general thing being that on its own it makes a great wall of fodder, relentlessly distracting, delaying and grinding down enemies in large masses, while offering pretty much anything another Hero might ask for in synergy. A lot of units, spawned in the middle of enemies or forming a large frontline, being entirely disposable in case you want to spam Unholy Frenzy. Making corpses in both quality and quantity, suitable for all corpse purposes including the occasional resurrection from a Priest or something. Having spammable abilities to recharge the mana for.
And I'd place the balance points as desperately needing at least one of these things to actually kill things because otherwise it'd run dry on mana, corpses or life before finishing a wave off.