I want vehicles that are unlike the standard turreted tanks we already have. They permit much more variation for missions. They can appear in larger numbers. And they interact great with infantry and tanks alike.
We have the Pwnzer IV and T-34 already.
Tiger vs KV are iconic but still very similar in most situations. They would only bring stuff to all tank-scenarios really. And that is limited.
Armoured Personell Carriers would permit mechanized infantry to make an appearance, finally. Carrying MG34 on top and supplies, offer rapid movement on the battlefield, machinegun support and extended operations. Can be a spawnpoint, too.
Marder II is a great idea. An open topped, tiny, lightly armoured, turret-less tank destroyer. That can still fire high explosive shells to deal with other stuff. Now that's a flawed vehicle with weaknesses that prevent it from being overbearingly powerful. It can be employed in frankly every type of scenario almost, and in more numbers than otherwise possible. Interacts well with infantry, vehicles and tanks. Also, imagine being able to look down from above and see the amazing crew animations at work. Wow.
Armoured recon would make an interesting choice and there are many different vehicles to choose from. And maybe could be made to play an interesting role of actually scouting out enemy, marking them on the map or HUD or be the only ones able to call in artillery. The other side would be most interested in shutting down these 'eyes' of the enemy.
Light early war tanks are a bit like the Tiger / KV-1. Just downgunned versions of what we already have.
The STUG series are at least more different, having lower profile and lacking a turret. It adds a bit more flavor. How about a proper assault gun that the germans used in Stalingrad?
STUG 33B