When I was playing using Vanilla:

1) I would never start a war, I would have to be forced into one.
2) Try to use a "historical" make-up of armies (but difficult post-Marian)
3) No extermination
4) Only specific settlements would be enslaved (i.e. Only "Barbarian" cultures)
5) Post-Marian armies must have at least two mercenary units.

Basically, the more you impead and slow your own expansion down the better.

Still starts getting boring once you get past 35 regions though.

Haven't had a chance to play my own campaign in RTR yet but I have high hopes for that.