I know this harkens back to older D&D versions AND a NWN expansion, so it's a niche suggestion/request but I truly enjoyed having pseudodragons and faerie dragons as options for familiars. I understand with the familiar system in 5E they would look vastly different (perhaps better as animal companions?) but seeing them incorporated into the game as a character option would be awesome. Their intelligence and unique abilities would lend itself to interesting gameplay options (both are capable of telepathic speech, at least one ability: pseudodragons have a poison stinger, faerie dragons have a lot-). The tabletop group I'm in let me incorporate a homebrewed pseudodragon into the game as a travelling companion to my warlock that we as group bought off a jerk, and it hasn't been game breaking in the slightest. But that's just a personal experience.
Perhaps I'm not alone in this opinion though?
