First for 1977 don't forget romanian forces. They were very strongly present at Plevna among others with 4 infantry divisions and 1 cavalry. They had very colourful units as the Dorobanti and the Elite Rifle Battalions.
For WW1 some french cavalry late war would be great. After all they took Uskub alone. And the long overdue French cavalry for the Cri;eam War.