I often play as a bard in Baldur's Gate or Fighter/Warlock, Fighter/Bard. So I hope party size will be bigger than 4 or they give me reason to pick bard.
Having bigger party would have been great (I mean, more banter is always welcome).
I guess the reasons behind the party of 4 are the system being primarily balanced around the party of 4, and the coop. I do not think we will get a bigger party, which sucks a bit (party of six means every encounter has to be rebalanced, to account action economy. DMG guideline is making them harder by 50% exp wise).
Luckily you don't have to force yourself into the archetypical party. Bard can fill the role of the rogue's, he is pretty much the skill monkey of 5e (you get expertise in a couple of abilities, which means double the proficiency for say lockpicking, and you add half of your proficiency too all abilities, you are not proficient in)
All you really need is most likely a cleric in the party, the rest is pretty much "pick whatever you want", so long you have someone with decent dexterity pick "sleight of hand" proficiency.
In my first playthrough, my warlock took the lockpicking duty :p