Party comp wise I'd say the most important roles to fulfill are:
- Ritual Casters, who would enable Longstrider and Jump ritual spells for your party. Having those makes a ton of difference all the way till act 3. Classes for that role are Wizard, Ranger and Druid.
- Party wide HP buffers. Unless you are not a barbarian, you'll always feel behind your enemies in terms of HP. Therefore having a Cleric or a Paladin to cast Aid spell on your entire party is very useful.
Technically... Neither of those are actually required in a party (Besides Enhanced Leap and Feather Fall. Though the number of times these make a massive difference can be covered with the millions of scrolls that are found everywhere)
You can swap a character in to buff all the members of your party with the "Until Long Rest" spells and then swap back your party member.
I'd also argue against the necessity of such things. Longstrider is nice, but you can do fine without it especially if you use the strategy of... Having your enemies come to you. (Which works well with a number of strategies and spells)
Aid is also unnecessary. I've never felt it be particularly noteworthy and would likely say that an extra Hold Person, Spiritual Weapon or Silence cast will provide far more health than Aid does via preventing damage and/or making enemies easier to kill (Thus preventing damage).
