If you're not shooting landscapes because of limitations of 35mm, don't generally think wide, and miss them enough, then a 2x3 century graphic, with some modern 6x7 or 6x9 back would be a decent compromise between negative size and portability.
If you shoot enough fashion to be frequently borrowing the RZ, then look for an older RB67/67Pro, and a second back. Your friend will appreciate it, and you won't have the scheduling issues.
Alternately, from where I'm sitting (upstate NY in the dark and damp), I'd take a week off and go somewhere photogenic to recharge, taking the Canon and 2 most commonly used lenses. (because you never know when you're going to drop one)