Considering Sephiroth was added partially to promote the Final Fantasy 7 remake, I don't think it's farfetched to presume that Leon could be added to promote the Resident Evil 2 remake. Also, from what I've heard, Resident Evil 8 was a pretty huge success. I watched a streamer play Resident Evil 8 and thought it was pretty cool; the villains are very eccentric and well fleshed out. For the most part, all the newcomers and DLC fighers in Smash Ultimate have been some pretty big names from very popular games. That said, I'll be surprised if Leon or another Resident Evil character wasn't at least considered. From what I've researched, Leon is practically the Cloud Strife of Resident Evil. Despite not appearing in the first title, he is widely considered the most popular character in the series.
I honestly wouldn't mind seeing a Resident Evil character in Smash, since I really like the series for the puzzle solving and the survival element. To reflect upon the Resident Evil series, I could see Leon having limited ammunition--and like Robin would be vulnerable until he restocked. Like Snake, he'd probably use lots of explosives. I personally would like to see the flamethrower and the bowgun in Leon's moveset. Also, just like in the Resident Evil games, Leon's movement could be hindered once his damage at 80% or so. He could be extremely dangerous at 0% damage, but once his damage % is high enough, he can easily be rushed down.
These are mine thoughts, at any rate.