Arslan/Daryoon is heavily implied. It's like Alexander the Great & Hephaestion with roles reversed.
The anime's character designer, Kamimura Sachiko, has done a series of BL illustrations featuring basically expies of Darun and Arslan. She denied any connection but it was pretty much a case of "well, just because one of them looks exactly like Darun and wears exactly the same outfit, and the other looks exactly like my version of adult Arslan, right down to the clothes, jewelry and weapon, doesn't mean it's them!"
Elam is extremely devoted to Narsus, and when Alfreed starts making overt moves on Narsus he becomes catty, as though he feels the need to defend his position as being closest to Narsus from the girl who might take him away.
Rajendra has said a few times that he likes Arslan, calling him a brother at heart, he uses the term "daisuki" when referring to the "like."
"Daisuki" is of greater romantic connotation than "suki," (which would be used when describing the like one would have to a friend or family) and Japanese couples will more often use "daisuki" when saying "I love you" to each other, as "aishiteru," the word that has the greatest romantic connotation of "like" is to only be said to the person you are going to be with for the rest of your life.
Elam has a more soft and gentle atmosphere around him when he's with or talking to Arslan, and when it is pointed out that Elam acts friendly with Arslan by Gieve, Elam is quick to deny it.
Arslan is shown to worry about whether Elam thinks of him as a friend or not, or even like him, Elam is quick to refute the belief.