I'm going to have to say Wedge. Guy had his own book & comic series pretty much with Rogue Squaddron (Although he was more of a side character in the Rogue Squadron game series) was a major player in many of the other books and comics (Even briefly captaining a Super Star Destroyer!), is uncle of the Fel Dynasty....etc but now he pretty much just shows up in the Aftermath novels, where he becomes Wexley's mentor, and in Rebels where he's yet *another* Imperial defector (The backstory in the old EU was somewhat more original, orphaned freighter pilot mentored by smugglers who joins the rebellion after his girlfriend is killed). It's revealed that he pretty much retired after Jakku, running an academy on Hosnian Prime, which means he's possibly dead.

Boba Fett also gets less love, as now I'm pretty sure he's still canonically dead post-ROTJ, but at least the Mandalorian kind of carries on his whole concept in a sense.