Here are some of unreleased/old cameos:
Allied Officer - Beta cameo
Centurion Dark Crawler (Now, Overlord) - Old cameo
Detriev (from Speeder) - Beta cameo
Golden Cosmo - A cancelled unit that has a jet pack and attacks with Tesla Zap (Obviously mixture of Cosmonaut and Tesla Trooper), costs $1200 and available with Soviet battle lab.
Gunpowder and Missile Drone - Another cancelled Yuri's extra robot aerial units.
Peaceful Hovercraft (from Speeder) - Beta Cameo
Super Yuri - A cancelled unit replaced by ????? Yuri. Also beta cameo.
Ahahaha super yuri xDD