T’Lyn’s story in Season 5 involves her and another character in an interesting way, and you see T’lyn embrace science and Starfleet more than I think people anticipate.
Until proven otherwise I'll remain on the Sokel-is-T'Lyn's-father boat and will assume this to be about him.
Hmm… Lower Decks tends to deal as reverently to Trek stars as it does one time TNG minor characters.
For a minor character cameo… I think I’d like to see one of the TNG S1 chief engineers. I bet they have a story to tell about working on the Enterprise then.
For a major character cameo… how about Chakotay? I’d like to see the Protostar prior to its launch and tie forward instead of backwards.