I'm making my first campaign a big one so any idea will help. if the pcs become the villain or hero what should oppose them.
A quest is a good way to start a campaign.
An army, religious sect, or any group that would not accept their point of views. An oppressed group that is now pushing their beliefs and becoming the oppressors is a good line. Reminds players that "alignment" is entirely subjective and depends on someone's point of view.
I do the same thing for my group my last 2 i made were really short but my group loved them!
on my first one it was just a pvp battle royale (my friends wanted to try it) they had fun with the competitive part of it. But my second one I made I put a twist in it; It was a zombie apocalypse setting and i made one of the players in the group part of an opposing clan. So at the end of the game they had to fight the opposer to beat it.
my really big campaign im working on right now is taking quite a while because i also create the npc's and the monsters (everything in mine are self made).
one thing that will help you the most (in my opinion) is to ask them what kind of setting they want so you have an idea for that and then work from there.
My group wanted a medieval D&D so i'm making one and I'm starting the group off by meeting in a tavern in one of my made up cities
my main boss is a necromancer.
my sugestions are to first see what would appeal to the group
then create some ideas for that
you don thave to have the full game made before they play it just some of it to see how the group likes it
then if they like it keep it going
if you have to improvise on the go you'll be suprised about how much you can create improvising
I would have an individual come and offer another a position, or have them deliver supplies and get ambushed by the person who hired them. You could have them rob a magical crown that rightfully belongs to one king but was stolen from another, you could have them be part of a rebel group, and you could have them be sent to another world and have to find their way back.
Just some ideas, never really played before.