So what are the implications then going forward for what a "perfect edition" of D&D would entail for the cleric, niche protection, and healing?
Well, looking back at the OP:
balance vs. niche protection: Each class will be specialized and fit a role at core. A fighter does things completely different than a wizard, and at times the fighter will outshine the wizard, and other times the wizard outshines the rogue, etc. However, over the course of a typical adventure, each class will have an opportunity to shine and be balanced with the other classes. D&D has always been a team sport. Every encounter doesn't have to be balanced, but you shouldn't have to wait levels to balance things out either (like magic users in 1e).
* class list: in addition to the core classes we all know (pretty much all the ones in the 5e PHB), also include a warlord and mystic class as core. Some changes to how classes have been done include:
--ranger: core ranger class is magic free.
--sorcerer: solely uses a spell point system instead of spell slot system.
--wizards and clerics have specialized schools.
It sounds like a bit of a murky compromise, balance v niche protection. There'll be some niche protection in that each class must do things differently, and put in variable performance that'll theoretically balance out over the course of an adventure (which is more ambitious than a campaign, certainly). "Spotlight balance," a zero-sum game in which playtime is divided among the players based on the situation matching their character's specialty (and means of delivering on that specialty).
Healing can hardly be niche-protected for the Cleric with the Druid, Bard, Paladin & Warlord all in the game. I suppose you could make them heal differently: Clerics with Cure Wounds spells, Paladins with Lay on Hands, Warlords with inspiration, Bards with Song of Rest, Druids with something like 3e Vigor, perhaps.
Clerics would be further differentiated from Paladins by, er, not using swords s'much, I guess, from Warlords by casting all kinds of spells, from Bards & Druids by turning undead...
...yeah, all sounds pretty plausible, conventional even.
