Alchemist: The Pathfinder Alchemist is a work of art, and would love a 5e equivalent, with a degree of versatility. I can see it fulfilling the herb healer that isn't channelling magical energies, the bomb throwing anarchist, and the Jekyll/Hyde potion drinker with ease.
The Artificer is another class I'd love refined and presented as an effective pc class. This is the class that I didn't like in 4e, but darmed if I know how best to do it for 5e. 3rd made it a prep guy, but that's kind of boring.
Book of 9 swords was a response for the broken spellcasters in 3rd. I'm not saying no, but show me the need for them.
Divine Classes: I am completely convinced that a balance between the 4e divine classes (Avengers, Cleric, Invoker, Paladin) and 2e Forgotten Realms speciality priests would create the perfect system for belief based classes.
I'd love it as a separate book, because honestly, clerics feel generic.
Tinkerer: I have a soft spot for a tech guy in a magic world. Hell I wrote a Artifex for palladium a while back that shows the raw potential of the concept. I ran a 2e Gnome Professor through expedition to the barrier peaks and it was awesome. Most likely this could be a reskinning of the artificer.
Truenamer is one of those ones I really don't have an opinion on, but am curious to see what comes out because of it.
Magic of Incarum classes need to be sold to me. I had trouble wrapping my head around the concept and maybe an explanation would get me on the side.
Psionics I want only because it's involves my favorite villains: illithids. I have yet to be impressed by any psychic system, but 4e came the closest. If you want to know what I want in a level based psychic system, check out my free download Dark Revelations book of arrogance.
-> I'd be okay with an ardent (aka psychic telepathy/tk person), eruptor (kinesis expert), and psychic warrior (bio-physiology and psychic weapons, whether ectoplasm or force energy based), split of the powers though.
Runesmith - really not sure, but won't say no.
Shapeshifter: We already have one, but I've been convinced for a long time, shapeshifting should have been it's own class.
Warlord: I've said my piece on Warlord on another post. if you want my opinion, read that. In short, I'm pro warlord (or for those that are terrified by the very name as an afront to your belief system, a Mundane Bonus Booster (MBB). )
As Sub Classes/Paths:
Acrobat/Jongleur: I'd love a really solid acrobat subclass of the bard/monk or thief. I blame a certain fury bikini on that one. .
I also think the following for the rogue makes sense: scout, shadow creeper, swashbuckler (to simulate everything from pirates to agile swordspeople)
Bounty hunter: probably a ranger or rogue subclass, but there may already be something out there.
Cavilier: this one I'm very much on the fence on. I think it's best to be either a fighter or MBB path, but opened to negotiations.
Charlatan: very much a sucker for this concept and it makes a great bard sub class
Greenwood Ranger: tree symbiote - oh heck ya.
Hivemaster: bug druid - oh heck ya
Jester: agree heavily this is a bard subclass.
Riddlemaster: this so needs to be a bard subclass yesterday.
Shadowmancer: should be sorcerer or warlock path. It's a great concept, but it's more a power source then a class.
Sha'ir: good gravy man - I want this one bad. Most likely a sorcerer or warlock subclass.
Swordmage: The 4e Swordmage is the single best gish in dnd, bar none. Because it's built as it's own class, it has it's own feel. I don't know if it's needed with the Eldrtich knight subclass, but maybe it can be modified to better match it.
Spellfire Wielder: some sort of reskinned sorcerer could have potential if balanced with other classes.
Warden: The Warden would make an awesome subclass for the Ranger, emphasizing the magical abilities. Same with the Seeker. IMO The magic-less ranger should be the base class and the others variants (because Robin of Locksley is the base that all other rangers are based on)
One Last Thing
I did a design challenge to build a spellcaster with rules as simplified as the champion fighter archtype on rpg.net, and providing a link with the results.
I think some of you will enjoy it.