Oh Yeah, This has been brought up before. Gssixgun has some good recipes, maybe he'll post them.
I use a few, One of his is "Lanolin Overload" This consists of Mitchell's wool fat soap, a bit of Musgo real shave cream, and 5 drops of glycerin.

I've added some Tabac shave cream to the Tabac soap.
SRD Sandalwood soap and Castle Forbes cedar and sandalwood.
Neither of these need help, it's just fun, and it adds something to it.

I haven't really mixed any opposites, I tend to keep them the same. If a soap is kind of flat, and it needs help, I'll use a shot of Kiss my face shave cream, and some glycerin to give it a boost....