I think marksmen needing to be broken down into smaller subclasses is long overdue, yes. Thus far they are still the only class to not have any subclasses under the official denomination list, and this is after they had an entire preseason update dedicated to them, suggesting that there is likely much deeper work to be done still. Part of the problem, I suspect, is that there's still a lot of overlap between marksmen in what they do, as autoattacking power grants a pretty broad set of strengths (e.g. waveclear, structure/objective damage, sustained teamfight damage, etc.), and is common across pretty much all members of the class. Because of this, I also think it's important that marksmen need to be split along what they contribute, rather than how they play: we already have caster marksmen and autoattack marksmen with very similar contributions, so separating them based on their ability usage risks creating further overlap along the way, which would be harmful to diversity of picks in the long run.
Personally, I think three of the classes mentioned by the OP could be suitable enough to split the marksman class into subclasses: as a class, marksmen supply damage over time via ranged basic attacks, and I think we can split that further into duelists, i.e. marksmen with above-average survivability who can kill single targets quicker, glass cannons, who can seriously affect multiple enemies when protected adequately, and utility providers, who round out teams with a blend of DPS and either strong utility or CC. I also agree that some AP do qualify as marksmen, so making the marksman denomination exclusive to AD makes little sense.
Based on the above model, here's how I'd divide the current marksmen:
Duelists:
- Caitlyn
- Corki
- Draven
- Ezreal
- Gnar (hybrid vanguard)
- Graves
- Jayce (hybrid artillery/non-specific fighter)
- Kindred
- Lucian
- Quinn
- Urgot (hybrid juggernaut)
- Vayne
Glass Cannons:
- Azir (hybrid battle mage)
- Jhin (hybrid utility provider)
- Jinx
- Kog'Maw
- Miss Fortune
- Twitch (hybrid assassin)
- Xayah (hybrid duelist)
Utility Providers:
- Ashe
- Kalista (hybrid duelist)
- Kennen
- Sivir
- Teemo (hybrid duelist)
- Tristana (hybrid duelist)
- Varus (hybrid glass cannon)
This is off the top of my head, so there might be some questionable champion-subclass pairings here.