I haved played d&d since first edition and the class structures were more true in previous editions. In the about 15 campaigns I have played in d&d 5e it has very rarely been a fighter/cleric/rogue/arcane caster set up, and I have normally played 4 man group. There is nothing you really need. You have short rest that gives plenty healing and long rest heals you completely. For locked doors and traps you dont need rogue anymore. Anyone can have the prof. If you do want some healing mid fight, a paladin, bard, druid or sorcerer (with subclass) will do the trick. Most spell casters gets decent AoE spells, and any warrior class will work for tanking (fighter, paladin, ranger, barbarian). Even other classes tanks just as well if you build them up for it.
Personally I am probably going to run around and solo most of the content with 1 character that is buffed by the other 3. I am just going around with a group sometimes to hear the banter etc. 2 more buffers would be nice, but having 6 characters in a turned based game is going to slow it down quite a bit and many will also be put off by the need to level up, equip and control so many characters.
