If there's a special table/rules for Stormseers, then there really should be a special table/rules for Rune Priests. And IIRC Emperor's Champion isn't specialty-based - anyone of any specialty can have the visions and become one for the upcoming battle. I think it'd be harder to model it as a long-term specialty rather than as just something that occurs for one major mission.
Honestly I know it's hard but not every Chapter needs to have a super-special rank. It's inappropriate for a lot of Codex chapters to have a more specialized advanced specialty aside from Honor Guard. Look at how big of a deal it was for the Ultramarines to create Tyrannic War Veterans, since the Codex doesn't mention them in its organization chart. Really if you're trying to invent something whole cloth and it seems off, chances are it's superfluous, doesn't fit, and shouldn't be added.
Firedrakes are pretty much "just" First Company Vets, for example.
Contrast this to, say, Silver Skulls Prognosticators, who are Librarian/Chaplains, or Iron Fathers, who are Techmarine/Chaplains, each of which fulfill a very nice role in their chapter.
Some of the Deathwatch-specific ranks (Forgemaster, Epistolary, Chaplain, Dreadnought) could be more generalized (as in less Deathwatch-specific) and used by any chapter as just part of the Codex as well.