I think the following factions should get Codices:
Ultramarines should stay attached to Codex: Space Marines with Raven Guard, Salamanders and White Scars.
Dark Angels, Iron Hands, Blood Angels, Space Wolves and Last Wall (Black Templars, Crimson Fists, Imperial Fists, rules for successors) should get their Codices.
Slightly off-topic, but I also think World Eaters and Emperor's Children need their own Codices and unique sculpts.
Exodites need to exist at all, and there needs to be a Codex: Genestealer Cults, but that's about it for Xenos armies.