Mine is Law & Chaos. Especially as a sandbox campaign, I want to give my players and party choices of outcomes. I start with the world at an inflection point where it could tip to either extreme. The players decide based on their alignments which way they want to tip it, or if they want to keep it in balance.

But this does create a problem if the DM wants to decide on a villain from outset of the campaign. If the players go evil, your villain is now an OP NPC that makes your PCs redundant. It's hard to guarantee you'll be playing a good campaign, if that's the kind of campaign you want to DM, or an evil campaign, if that's what you prefer.

I'd say if you have a more plotted campaign, maybe you want your campaign based on a theme, like, "Love conquers all," or, "Cheaters never prosper." But for more sandbox campaigns, a pair of opposites without making any claims about them might be enough to get you started without getting you locked in.

I don’t usually have a theme for the campaign 

Quote from Jack727 >>

I don’t usually have a theme for the campaign 

 Same for me. I just have a few story hooks for the players to bite at, and a couple of thoughts on how to develop the story from that.

when i write my sandboxes, i tend to have an idea for a boss, and give the party reasons to hate them throughout. the secret is to make a BBEG that no matter the party's goal, their goal always conflicts it. if the party is good and the BBEG is evil, no problem! even if both are evil, 2 evils dont usually get along. if 2 people want to take over the world, that directly conflicts. if person 1 wants to blow everything up, and person 2 wants to scam and blackmail people into giving everything to them, then person 2 cant accomplish their goal if person 1 does.

"Should all D&D campaigns be about [doesn't matter what goes in here]?"

No.

Moreover I don't think it's even necessary for D&D campaigns to be "about" anything other than the characters making their way in the world they're in.  Sure some DMs will structure a campaign around a theme or conflict (and that's all the "balance" about is, because in the OP's own formulation balance is just one sort of position one can take in regard to a central conflict).  Others won't.  Maybe as the characters interests and potentials are realized a central them or thrust will emerge.  But it doesn't have to.

My main game, the one I spend the most time on, is at a cross roads.  If they cross through a certain shadow crossing and return, they may find themselves in a sort of deconstructed Tyranny of Dragons which relies more on prior edition Cult of Dragon lore as well as homebrew dragon lore.  They may wind up on a fragmented comet world trying to be horses and men for a King whose domain has gone very Humpty Dumpty.  They may become vassals of an Ancient Blue Dragon freed from the curse of being a duck.  They may spend their lives tracking down ancient texts.   They may uncover various interplanar manipulations of the prime material, and in the process may learn where soul's come from.  

But none of that, if any of it happens, is what the game is about.  The game's about a Dragonborn bard trying to overcome her fears to more bravely honor her draconic heritage, a half elf bard whose lack of place in the world grants him insights into the workings of the world and also puts him on the fence as to whether use those insights nobly or cynically, an inquisitive Tabaxi who just wants to see wondrous things, a Tiefling champion who fights for this party because she doesn't really have anything else, and a Hobgoblin wizards who values this party ... for their uses.  The game is about learning about this party and playing their growth, maybe witnessing their deaths, and the new characters they may meet along the way.  And it's fun.  I don't know if all D&D campaigns should be enjoyed this way, but there's lots of ways to do it.