4E found a solution around this flavor problem:
Intraclass Feats, a sort of inside out Multiclass system. You could give up a bit of a subclass resource to access a bit of a differen't subclass's resource within your own class. So for example, a Protector (the 4E Player's Option: Heroes of the Feywild Druid variant that is the predecessor to 5E's Circle of the Shepherd) could take a feat to trade a use of Summon Nature's Ally for a use of the '08-Druid's Wildshape). There's a similar feature for the 4E Player's Essentials: Heroes of the Forgotten Kingdoms Druid variant, the Sentinel (a Beastmaster Druid harkening to 3E's Animal Companion) that they can give up to get a use of Summon Nature's Ally or Wildshape – it's a resource feature that props up their animal companion's abilities. This all still cost a feat and cost a small amount of your primary subclass' resource feature, but it was a small price to pay to answer this flavorful question elegantly!
Of course, 4E's feats were MUCH smaller in scope than 5E's feats, which are essentially non-classed class-features, so a 5E version of these feats would probably want to ixnay on the downsides; the opportunity cost of taking this feat over a different feat or over an ASI is cost enough already! But it does highlight the solution for 5E, some of which 5E has ALREADY implemented.
Tasha's Cauldron of Everything has the Superior Technique option that Fighters can take instead of a Fighting Style Feat –– this grants them a Battle Master SD and Maneuver. So either it's a useful option for Battle Masters to bolster their usages and versatility, or else it's a way for other Fighters to access the subclass features. 2014 Player's Handbook also had the Martial Adept feat that grants ANYONE access to a small amount of the same battle Master resources. Superior Technique is actually a bit smaller than Martial Adept because the 2014 thinking was that Fighting Styles are worth half a feat, but this was reassessed in 2024, and now they're considered close enough that a Fighter, Paladin, or Ranger may forgo a normal feat to access another Fighting Style. Tasha's Cauldron of Everything also granted the Fighting Initiate feat that grants takers access to any Fighting Style from the Fighter class (I'd house rule this to be any generic Fighting Style Feat, thus locking off Superior Technique, Druidic Warrior, and Blessed Warrior given that the latter two were kept out of the feat category as alternatives to gettting a Fighting Style feat). I'd note that Martial Adept is officially considered a Legacy option and while Tasha's generic Fighting Styles (and Unarmed Fighting) were ported over to be Fighting Style Feats, the Fighting Initiate feat was not – likely because the Weapon Mastery feat already offers some of the conceptual space of Martial Power to characters that aren't classed as Barbarian/Fighter/Paladin/Ranger/Rogue.
But in any case, these are just some options that offer non-Battle Masters some intraclass features from the BM subclass. Likewise, Magic Adept (Arcane) gives some Eldritch Knight flavor to non-Eldritch Knights (and bolsters EK's options a lttile), and there are a few other feats you can take to lean into that multi-subclass kit idea. There's also the Telekinetic feat, which gives a taste of the Psi Warrior to other characters (or can bolster th Psi Warrior's options).
Warlock is another example where a bunch of its Invocations could thematically reflect other patrons. For example, Grasp of Hadar from Xanathar's Guide to Everything is very conceptually tied to the Great Old One Otherworldly Patron (given that this is the Patron that ate 4E's Star Pact builds for 08'Lock, Hexblade, and Binder), but is available to any Warlock. Unearthed Arcana experimented with Invocations with specific subclass prereqs, but XGE, TCE, and BOTH versions of the Player's Handbook keep the Invocations Patron-prereq free, tying the prereqs instead specficially to level and/or Pact Boon (which 2024 PHB made into their own series of Invocations for those who want a bit more versatility across the Pact Boon options – another example of this creative philosophy at play).
I'd note another similar change from 2014 PHB to 2024 PHB is letting Paladins and Rangers access any of the generic Fighting Styles that Fighter could always access; even when they expanded the styles in TCE they still tied the expansions somewhat arbitrarily to either Paladin & Fighter or Ranger & Fighter (with Fighter getting 2 further exclusives and Paladin & Ranger each getting their exclusive option per above).
It just was becoming increasingly hard for WotC to justify locking Archery, Two-Weapon Fighting, and Thrown-Weapon Fighting from Paladins and to lock Great Weapon Fighting, Protection, and Interception from Rangers. I think 2014 was just too caught up with the idea that Rangers -ARE- the Bow or 2WF class as they were in 2008's PHB (despite the fact that even in 4E, Fighters and Barbarians, amongst others, had gotten build expansions to let them do those concepts as well). Similarly, the idea of a Great Weapon Ranger made them ask why isn't this a Barbarian? These fighting styles binding weapon choice were carried over assumptions of past editions, especially out of a desire to protect the Ranger's "niche." I would note that in the name of preserving the Ranger's ranged-attack Gishy niche, I think they didn't go far enough with Paladin's access to Archery & Thrown Weapon Fighting, because Smites still require Melee attacks, but at least now the build exists as an option (trap or otherwise).
I can see how they'd think that it doesn't make AS much sense for Fighters & Paladins to get access to Druid cantrips or Fighters & Rangers to get access to Cleric cantrips, so I understand keeping those two options as class-specific choices to help enhance the narrative ties between Paladin and Cleric, and between Ranger and Druid, specifically. Though personally I think if an Oath of the Ancients Paladin wants to take Druidic cantrips instead to help lean into that flavor early, why would I lock it behind the Ranger? And likewise for a Monster Slayer Ranger (taking a bit of Cleric cantrips to give that Van Hellsing vibe)?
In a perfect world I would have made "Arcane Warrior" and "Psionic Warrior" Fighting Styles and taken the cantrips out of the Eldritch Knight subclass. But I also understand the idea of wanting that to be part of the EK so that they can take a separate Fighting Style. I just would have suggested that Magic Initiate (Arcane) would be their 1st level Eldritch Knight flavor, then an Arcane Warrior fighting style would be their 2nd level flavor, and then the actual subclass would take it from there. I also think Arcane Warrior idea would be applicable to Arcane Archers as well as to Oath of the Watchers, Oath of the Noble Genies, & Oath of the Spellguard Paladins (and for Rangers such as Drake Warden, Horizon Walker, Gloom Stalker, and Fey Wanderer). I think this is probably my biggests ask of 5E's next errata / rules expansion / 10-yr iterative ruleset, honestly.
If we turn our attention to Bards, there are ways to lean into the other Colleges' narratives using feat choice as well. And over at Clerics, there are definitely feats as well as direct spell choices that can help prop up the idea of a Cleric with multiple Domains (though I honestly think we need a "Pantheon Domain" for the Cleric to give that generalist option like we get from the "Scribe Tradition" for the Wizard). The Scribe is this option for Wizards too, though I'd note that every Wizard can take advantage of the spells from the other domains, they're just not quite at good at doing so.
Spellcasters in general are a bit easier to borrow intraclasswise, unless the subclass is offering spells prepped expansions well beyond the normal flavor of the class. But we can take the Fighting Style and Combat Superiority feats & options above as a guide for this sort of kit-bashing to give us the dials and levers needed for any particular class's subclass list. The trick is making sure doing so doesn't crack open the class to EVERYONE else without a significant Multiclass level dip (hence why Subclasses are 3-level deep across the board in 2024).
