Starting my recent campaign I defined the three basic types of adventuring parties as heroes, vagabonds, villains, and agents. The first three correspond more or less to good, neutral, and evil parties, while the third category if for a group where each individual has a strong personal motivation to be there working together (such as working for the same organization or patron).
I explained how the different types of parties would act, making pretty clear where the line is between hero and vagabond, and between vagabond and villain. I told them that a party of villains is off the table for this campaign, with a little of explanation to the newer players as to the dynamics of evil parties and how you have to plan the campaign around the idea for it work (other experienced players did as much as I did in explaining it to the newer players, so that was helpful).
I told them that they can discuss amongst themselves and choose where they want their party to fit in that classification. They ended up going with a combination of vagabonds and agents. 3 of the characters were determined to have basically been a group of three traveling friends. The other 3 characters didn't have any party connections, but the whole group was working for the same organization and had some positive connection to a particular NPC. (It's starting with Lost Mine of Phandelver, and the characters were all hired by Neverwinter as supplemental forces with Gundren Rockseeker as a shared friend).
Then, to make it even more up to them, while still maintaining my DM interest, I explained the rules used (everything in the PHB is allowed for this campaign, with a 2-page sheet of house rules and clarifications that won't matter to half of the characters); then explained how to make their party.
I instructed them that each player needs to think up a few different character concepts. Type them up (online VTT) and place them under their name where everyone can see them. Then they could create a party by choosing which characters they were going to play to make a good party.
It worked like magic. Everyone (with one exception) is on the same page, they made a party that covered all the bases (rather than a mismatched group like you get when everyone shows up with a single character they *must* play for that campaign), and the best part is that I didn't have to even look heavy handed.
They pretty much did all the party creation work, and then I checked back after a while to see what they came up with. Then we dismissed for players to create characters, and planned to start up with our characters on the next session.
The one exception is the newest player who seems to really want to play a villain, but has tempered it a bit, and I have hopes her character is going to fit in fine. If she doesn't, she'd feel pretty uncomfortable since the group all decided together what type of party to play and how they were going to put it together and it was all clearly defined.
And as the DM you must be clear on your expectations also. If you are running a game about heroes from the classic classes and races, then you need to tell them that's what the game is about, and if they aren't on board then you don't run it (or run it for those who are on board).
That's my new standard. Figure out what you're doing before you create a party, or the characters that compose it. Then everyone knows what's going to be the game, and knows what they are signing up for or not.
