I vary rules between campaigns and one-shots, using what best supports the theme and play experience I'm going for. I've used rest and XP variants plus Hero Points. I've also included firearms in a couple of adventures. It's all worked fine because I was very choosy in picking only those options that supported the particular theme I wanted to see at the table.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.