I personally think that if Sylvanas had never fallen during the Invasion of the Scourge, her and Varian would have been extremely close friends, if not there being mutual respect.
Varian and Sylvanas were both willing to sacrifice their life for the defense of their kingdom, with Varian dying so the rest of the Alliance military could get away and Sylvanas dying in a vain attempt to protect the city.
They’re also both military strategists, with Varian having led the Alliance side of the campaign in Northrend, and Sylvanas being High Ranger General as well as orchestrating all of the military campaigns during the fourth war.
Not to mention, if Sylvanas didn’t fall to the hands of Arthas, the Blood Elves probably would’ve never joined the Horde (as it was only due to Sylvanas, who then became the leader of the Forsaken, vouching for them).
These are just my personal thoughts, but I can definitely see Varian and Sylvanas having a healthy friendship with eachother if things had turned out differently.