Arslan Senki

Both are by the same author, although one is a medieval world with light fantasy and one is a space opera setting. While Legend of the Galactic Heroes follows tactics and political situations more closely, the Arslan OVA follows Arslan's group as it becomes an army. Both portray each side of the war going on in grey shades, although Arslan shows the opposing side in more of a negative light and is a bit simpler in terms of morality. 

The source materials for both are long novels series, yet both are also similar in the sense that their anime counterparts are incomplete. However, each of them definitely capture the essence of their expansive fictional worlds modelled after historical periods and places, and the grand scale of the storylines. The intricacy of the plot, albeit unexplored due to the lack of completion, and the diverse cast of characters also carry a similar vibe. Political intrigue and different 'sects' of characters are prevalent in both. And they contain fantastical elements, though more so in Guin Saga than Arslan Senki. And both series have rather melodious ending themes. 

Medieval Fantasy Saga about party of brave adventurers. Battling kingdoms history line hides dark magician conspiracy.