I think the classes on each team should match based on specialty/role, using Space Marines as base template.

Scout = Heretic (Chaos) / Gaunt (Tyranids) / Kommando (Orks) / Ranger (Eldar)

Devastator = Havoc (Chaos) / Hive Guard (Tyranids) / Loota (Orks) / Dark Reaper (Eldar)

Assault = Raptor (Chaos) / Genestealer or Shrike (Tyranids) / Stormboy (Orks) / Banshee or Corsair (Eldar)

Tactical/Specialist = Chaos Marine / Tyrant Guard (Tyranids) / Slugga or Shoota Boy (Orks) / Dire Avenger (Eldar)

Apothecary = Plague Surgeon (Chaos) / Neurothrope (Tyranids) / Painboy (Orks) / Spiritseer (Eldar)

Librarian = Sorceror (Chaos) / Zoanthrope (Tyranids) / Wyrdboy (Orks) / Farseer (Eldar)

Veteran = Champion (Chaos) / Warrior (Tyranids) / Nob (Orks) / Exarch (Eldar)

Dreadnought = Chaos Dreadnought or Hellbrute / Carnifex (Tyranids) / Deff Dread (Orks) / Wraithlord (Eldar)

Terminator = Chaos Terminator / Broodlord or Tyranid Prime / Mega Armored Nob (Orks) / Wraithguard and Wraithblade (Eldar)

This will require overhauls of current class structure, points cost, and balance. But at least it will be accurate and equal in terms of comparison to Space Marines.