I get that from a PnP purity perspective, but I’m more concerned about gameplay here. I’m hoping someone can offer reassurance that we won’t need to pick tank, thief, healer and spellslinger to be effective in combat. For me, a huge part of any good CRPG is experimenting with team composition and cool class combos.
In 5e many class can cover the same role if built properly.
a ranger or a bard can be the typical stealth guy. Even a fighter can (a dex fighter in light armor and finesse/ranged weapons).
Rogue/bard are just the best class for steath/stealing/picklock (they are the specialist class because they have expertise = double proficiency with some skill)
Any class can be the group “face” in particular one of the many cha classes.
Paladin, bard, warlock, sorcerer are prefect for that. A rogue can work too.
Bard is the best as face.
Healing in 5e in mostly out of combat, all class can use hit dice for self heal during a short rest(a pool of 1 hit dice per level). Long rest restore all hit points and half of the hit dice pool.
so what healer do in 5e?? They combat heal.
Combat heal: when someone go to 0 the healer heal some hp so they can keep fighting.
it’s not a mmo, healer don’t have to keep party members full.
Life cleric is the best healer, all clerics generally prepare some healing.
Druid, paladin, bard can also heal.
Sorcerer/warlock can heal with the correct subclass.
fighter have some self-heal. A monk subclass can self heal.
There is no real aggro mechanic in 5e.
some class can do limited stuff for attract attention or punish enemies.
Attack of opportunity exist.
The feat poleweapon master allow to hit who come close to the character.
barbarians have resistance to phisical damage (half damage) so they HP tank: they use reckless attack so they have advantage but also enemy have advantage against barbarian (so enemies tend to focus the easy to hit barbarian)
Moon druids: bear tank
Paladins of course they have auras for superior saving throw, in addition to protection/healing.
Fighters have self heal, reroll saves, and have more feat/stats.
clerics can be good frontline too.
all classes can be good damage dealers.
martial classes are best at single target, paladins and fighters can go nova and massacre a boss.
Casters are great at aoe damage (fireball with 8d6 outperform others damage spell) and control, generally caster are the problem solver.
TL; DR in 5e you can play with a mono class group or mix with no real limit or problem.
4 bards? Can do all
4 paladins? They suck at ranged but no problem.
4 clerics? A-men Easy mode.
4 wizards? If you win initiative there is no hope for the enemy.
4 warlocks? I guess you really like eldritch blast.
Etc