The problem with AIs is that (usually) they do the same thing every time. Even if you randomize strategies at the start of a game, once you figure out which AI enemies are using which strategy you've got the drop on them.
I like to play solo, so I always decide at the start of a game what my empire is going to do and then stick with it.
A favorite role-play is to pretend to be defense-only isolationists. This limits expansion and gives the AIs a head start. Inevitably, one AI gets very aggressive and demanding. Maybe they crash a warp point and raze one of my colonies. My people become incensed and outraged. Nothing but unconditional surrender will satisfy them. Depending on treaties, this can snowball into war with multiple AIs, or my people can go isolationist again.
Another role-play is to decide that my people are blood-thirsty savage xenophobes, who immediately declare war on anyone they meet and glass every colony they encounter.
Another role-play is to be as pacifist as possible (short of surrendering) while still being expansionist. Maybe use intel a lot also.
Another fun thing is to make two human-controlled empires at the start. One is normal; the other is The Ancients. You use cheat codes profusely for the latter to get them super-advanced. You seed the galaxy with undefended supercolonies, computer-controlled uberships, maybe even some heavily-defended ringworlds. Then you have them abandon everything and retreat to one system with a sphereworld, knocking out all warp connections. (This can all be done in a few turns. You can temporarily change all empires to "human-controlled" and have them do nothing until The Ancients are set up.) For the rest of the game, just hit "end turn" every time The Ancients comes up. (Impt: don't use any ministers for them!) See how long it takes you to contact The Ancients. Toss a coin to see if they're happy to see you.
(Oh, also, give them high intel points and set it all to defense. Otherwise an AI might use AI to get their sphereworld. Unless you WANT that to happen....)