While I'm not sure I have a true "must see" list, I can think of a few neat ideas that would be worth exploring.
Wizard of the Spiral Tower--I dunno if this existed prior to 4e, but it was an interesting way to mix a bit more "martial" into the Wizard, rather than mixing a bit of Wizard into the Fighter (as the EK subclass does). I dunno how you'd need to handle it, but just as the Fighter has essentially the same core package with a handful of spells (they're essentially "one-third casters" instead of "half" or "full" casters), it could be neat to have a Wizard that gets a taste of Fighter stuff. Maybe a special, magic-infused set of maneuvers? I dunno. While there are many "gishes" in 5e, there's only one base class that casts spells from intelligence (Wizard) and it doesn't really do the "I fight in melee" thing. (Abjuration comes close, but it just makes the Wizard a bit more durable, it doesn't do the 11th-level damage boost that nearly all meleeist (sub)classes get.)
Lion of Talisid--I freely admit that this PrC was a bit crazy (anything you could MC as a 3.5e Druid and potentially gain power would be), but I played one through NWN2 (via fan patch) and thought it was a fun idea. Something that blends a bit of Cleric or even Paladin into the Druid could be neat. Again, no idea how to balance it properly, unless it requires that you're a Moon Druid, which I dunno if that's an acceptable type of requirement--but if it delayed your CR progression and gave you other bennies in exchange, it could be acceptable.
Knight of the Weave--We've already got Paladins that can smite from their spells, but I thought this PrC was, again, a neat way to blend two different concepts (arcane caster, divine warrior) into an interesting whole. Simply having it require that you can cast 2nd level Arcane and Divine spells should be sufficient cost. Even if you went Cleric+Wizard, you couldn't start it until at the earliest 7th level. Bard+Paladin or Sorcerer+Paladin would syngergize better, but leave you with even lower max spell level and require another two levels to get in, so again, seems relatively balanced.
More or less, I feel like PrCs are best when they either do something like the proposed (although poorly-written) Rune Scribe class--they create a new mechanical space with a meaningfully different "take" on current mechanics--or which work by creating a thematically and mechanically meaningful synergy of two distinct disciplines. So, as another example, I could see a wholly new (AFAIK?) PrC that blended the unique mechanics of the Sorcerer and Bard (sorcery points and bardic inspiration, respectively) into something with a touch of both, yet also new too: call it a "Bloodsinger" or "Soulsinger," and have it be specifically better at assisting ally spellcasting (which, unless I'm mistaken, bardic inspiration is largely unable to do--only spells with attack rolls can benefit, and those are few and far between).