I don't see Young Link coming back to Smash so easily, considering Toon Link pretty much inherited his quirks as a Link clone.
Both share key aspects in terms of moveset and aesthetic design in relation to Link:
1. Faster, but weaker and with lower reach
2. Spin Attack on the ground deals multiple hits
3. Down Smash has the first hit connect into the second
4. Sword hits use a smacking sound effects instead of slash
5. In Smash Bros. 3DS/Wii U, Toon Link can use the Fire Arrows as a custom move
However, there are differences between Young Link and Toon Link:
1. Toon Link's up Smash deals 1 hit instead of 3
2. Toon Link doesn't use kicking moves (meaning the neutral and back aerials and the forward and back throws are different)
However, considering Falco, Roy and Ganondorf also got similar moveset and aesthetic changes in games released after Melee to make them less clones, you can argue that Toon Link's moveset is a similar treatment to Young Link's moveset.
Characterization-wise, they are different characters, which gets reflected in stuff like taunts, win poses and trophy descriptions, hence why Toon Link was introduced as a newcomer in Brawl. Gameplay-wise, he's essentially a semi-cloned Young Link.
The only thing I could see in regards to Young Link returning would be diverging his moveset in a different direction from that of Toon Link. Perhaps, the Fierce Deity Mask could come into play as a Final Smash, for example, as well as some different animations and other such similar changes to normal and special moves. Sakurai may just see too much overlap to include both Young Link and Toon Link at once, though.
And before someone brings Hyrule Warriors regarding on how the three Links could be so different: don't. Smash and HW are two different games, belonging to different genres, with different inception time periods, development histories and design philosophies. I don't think it's fair to compare both games on how they handled the same 6 characters in common (Link, Zelda, Sheik, Ganondorf, Young Link, Toon Link).