I really don't see Lyn being promoted to playable status, because I think the reason why is because she's not the only main character in FE7. There's Eliwood, Hector, and the Tactitian, and they along with Lyn share an importance to the plot. You can't one of them playable without the rest of them, which was why she became an assist trophy. I'm also seeing a similar issue with Chrom because he's not the only main character in Awakening, as there's also Lucina and the Tactitian. He can't be playable without including one of the two, unless Sakurai finds a way to incorperate the pair up mechanic from Awakening in SSB4 with Chrom and Lucina/Tactitian.