I would argue that the 4 CLASSIC CORE D&D classes - fighter, rogue, wizard, cleric - should have more sub-classes than the other classes because they are (a) iconic, (b) have had more time to foster variants, and (c) anecdotally seem to still be the most frequently played. This argument is already halfway carried by the PHB which presents 7 sub-classes for cleric & 8 sub-classes for wizard. We could ask: Why did fighters and rogues get the short end of the stick compared to their magical CLASSIC CORE class brethren?
Even while keeping in mind that the PHB is not the end of it for subclasses, I totally agree with you. They already had at least 5-6 Rogue subclasses during the playtest, and they had a couple more Fighter subclasses too, although some of their abilities were carried over to other areas of the game, and so they would need some additional design nowadays.
At least for Rogues, we still don't have a subclass that fits well with the most iconic roguish character in fiction, which is Indiana Jones. He's not a thief, not an assassin, not magical, and not much of a fighter either (i.e. Swashbuckler), yet he's the most Rogue of all
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Back about the topic of
identity, it's true that Fighter subclasses are quite 'generic', but there are others who aren't significantly more specific either. The Berskerer is just a Barbarian++. Lore Bard and Valor Bard are just Bards, a bit tilted towards knowledge of combat abilities. Life Cleric heals more, War Cleric fights more, Knowledge Cleric knows more. The Hunter is quite a lot generic, the Beastmaster has a pet. Paladin of Devotion vs Paladin of Vengeance, in many ways still paladins, only different methods. They all have a bit more flavor than a Champion or a Battlemaster, but not much more.
Others are indeed more evocative. Paladin of the Ancients, Great Old One Warlock, Totem Barbarian, Storm Cleric...
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IMHO we shouldn't underestimate also the power of names.
"Champion" really is a huge miss. Champion of who or what? It could have been the Champion of a Lord or a Cause, but the abilities do not reflect that at all. There is nothing that 'activates' or 'works better' when the Champion is fighting for her faction or cause.
I stand on my opinion that if they had called it Veteran, it would have had already an improved image and more identity, even with its current bland abilities. Immediately my mental image of the character adds scars, seasoned body, wise gaze, conquered trinkets and badges... And it would also fit the abilities better, because they all represent durability and efficiency, which is exactly what someone gets by doing more or less the same thing over and over i.e. being a 'veteran' of something (the word's original meaning is "aged").