I'd love party size to be increased to 5 (or 6, though that really is quite a bit for a game like this). I believe that Wizards of the Coast has said that the typical D&D party is 1 DM and 5 players, on average. For that reason I'd like to see a bigger party (because that is what I see at my tables).
That said - in 5e you don't "need" certain classes in the same you did in the last 2 editions. In 5e, you even have multiple options for a "healer" character - cleric, druid, bard, paladin and even Sorcerer and Warlock (if you use options from Xanathar's Guide like the Celestial warlock patron). A "tank" role can be filled by lots of classes, including some Clerics and Monks. Most classes are capable of high damage.