If you were designing a new 5e subclass off of one of the many prestige classes that 3e had to offer, which ones do you think would make a good 5e subclass? The 3e Prestige class can be from WoTC or a 3pp such as Legendary Games.
Thematically, the Dwarven Defender subclass of the Fighter would be rather easy to put together I'd imagine. The biggest hurdle would be just what mechanics could be given to the Defender that one wouldn't already be able to put together with a Battlemaster.
The Shadowdancer prestige class would also probably work... but the first step would be deciding what class it was a subclass for. Monk already has Shadow so it wouldn't probably be that... and Bard now has the College of Dance so one would probably not go in that direction either to avoid doubling up. Which means the most likely class to have a Shadowdancer subclass would be Rogue. So at that point it's a matter of figuring out how to make a Shadowfell-themed Rogue subclass that doesn't step on the toes of the Warrior of Shadow or College of Dance.
Other than these... we already have Arcane Archer, Arcane Trickster, Eldritch Knight, Assassin, and Horizon Walker in the game. Archmage and Hierophant are just really high-level Wizards and Clerics so I don't know how necessary these subclasses actually are. Dragon Disciple has been kind of usurped by the Dragonborn species and the Draconic Sorcerer origin so that might not be needed anymore. Duelist, Loremaster Mystic Theurge, and Thaumaturgist would be the other 3.5 prestige classes (at least according to the Hypertext d20 SRD), but the question would be how important those would be thematically to the game?
PrCs
Arachnomancer/Verminlord: Wizard or Sorcerer, gets powers to talk to vermin, swarm controls, and buffs to their bug-based spells plus a couple of 1/day "spells".
Arcane transformation: Green Star Adept/Acolyte of the Skin/etc. Sorc subclass where the main features are taking on more aspects of your power source beyond just "you get free Mage Armor and +1hp/level."
Bear Warrior is about 70% covered by Beast Barbarian since base stat increases don't matter so much.
We don't have any real psionics, so Psionic Fist and the related psion PrCs are out.
Force Missile Mage, where you get upcasting options that add riders to your magic missiles and a buff to Mage Armor, Shield, and Bigby's Hand. Still not sure this can be good.
Runescarred Berserker:
Features:
Level 3
Runic casting: You may ritually scar your skin with magical patterns called Rune Scars. You may activate a Rune Scar as a bonus action to cast a spell upon yourself, even while you are raging. Activating the Rune Scar requires only an effort of will; no verbal, somatic, or material components are required. Each Rune Scar may be activated once per long rest. If you wish to cast the same spell twice in one day, you must have selected it twice. Some Rune Scar spells require Concentration. You may mantain concentration on these hile raging. If your Rune Scar spell requires a saving throw, the save DC is 8 + your Proficiency bonus + your Constitution modifier.
You may take the time to remove, bandage, and replace an existing Rune Scar with a new one of the same level upon level up.
You gain proficiency in your choice of either the Arcana or Religion skill.
Spells available at level 3: (choose two)
Cure Wounds
Darkvision
Divine Favor
Earth Tremor
Protection From Evil & Good
Spider Climb
Treat these spells as cast from a second level spell slot for purposes of upcasting and dispellation.
Level 6: (choose two)
Alter Self
Enhance Ability
Invisibility
See Invisibility
Warding Wind
Treat these spells as cast from a third level spell slot for purposes of upcasting and dispellation.
You may now activate a Rune Scar as part of the bonus action you take to enter your rage.
Treat these spells as cast from a fifth level spell slot for purposes of upcasting and dispellation.
Level 14: (choose two)
Antimagic Field
Heal
Investiture of Flame, Ice, Stone, or Wind
Mind Blank
True Seeing
Treat these spells as cast from an eighth level spell slot for purposes of upcasting and dispellation.
A level 20 Runescarred Berserker NPC was successfully able to duel a 15th level sorcerer/Adult Silver Dragon solo in a game I'm wrapping up. It was close, but shutting down spellcasting is pretty great for a martial.
Luckmancer/Luckstealer: Why isn't there a rogue class that plays with fate? So far the max luckmancy you can do is a halfling diviner with Bountiful Luck and the Lucky feat. Most of the rogue's power is in the chassis, not the subclass anyway.
Master thrower, for the bouncing shot and similar abilities. It'll be better than Arcane Archer.
Juggernaut (originally warforged, but expand to barbarians): Better knockdown, add an overrun ability, enhanced Charge ability, size increase, crit immunity. Possibly covered by Giants barbarian.
Stonelord: Originally a Martial PRC (I saw it mostly for fighters) that was almost good, but too limited. Slow self-healing, and you acquire a set of spells over time including meld into stone, summon earth elemental, stone shape, and earthquake. Some of the spells are too high to replicate with Eldritch Knight, and I'd probably want to add some free riders when dealing with enemies standing on the ground. This is still possibly too niche. You could even make it a barbarian subclass with earth magic.
Base Classes
Dragon Shaman - an auramancer focused around AOE damage. This would probably need to be a base class.
Here is a List of all prestige classes. Looking up the prerequisites and the descriptions gives the general idea. Most of them aren't really worth much or are just OP.
I'd like to see them visit the divine assassin theme, there were several prestige classes that covered it but I can't really remember which ones I liked best. In any case it Rogue subclass that gets divine magic/abilities.