Why not use something like Band in a Box. With Band in a box, I create full arrangements -- minus the parts I want to create -- and then jam on top of that. I've done things like create twelve bar blues, arrangements using standard progressions like I IV V VI, I IV V, etc and then would go back and add the II or III. Repeat the progression using another style -- country, rock, ballads, and you get the idea.
Why not use something like Band in a Box. With Band in a box, I create full arrangements -- minus the parts I want to create -- and then jam on top of that. I've done things like create twelve bar blues, arrangements using standard progressions like I IV V VI, I IV V, etc and then would go back and add the II or III. Repeat the progression using another style -- country, rock, ballads, and you get the idea.