Your .308 covers a lot of territory. So if you want something smaller for deer, look to a .243, .260 or 23-06. If you want a varmint gun a .223 or 22-250.
If you want more power you'd have to go up to at least one of the 7MM mags or .300 Win mag to have any great power increase over the .308, or even one of the big bores, just for having something different. Something like a 45-70!