Let's think about this realistically. Lyndis comes from a portable game, and what's more, he was secondary among its main characters. In the years till now and SSB4, it's very unlikely that she'll maintain the popularity needed to even become another assist trophy.
That said, I think of all the ATs, that Shadow and Gray Fox have the best chances. Gray Fox's Cyborg Ninja outfit has become a staple of the MGS series, and Shadow has a large fanbase in the Sonic fandom.
Yes, I know. The "internet" hates Shadow, but he's still pretty good with general audiences. If Sonic Channel's polls are any indication.
That said, I think of all the ATs, that Shadow and Gray Fox have the best chances. Gray Fox's Cyborg Ninja outfit has become a staple of the MGS series, and Shadow has a large fanbase in the Sonic fandom.
Yes, I know. The "internet" hates Shadow, but he's still pretty good with general audiences. If Sonic Channel's polls are any indication.