There are some pretty great Dobro (resonator guitar) players in bands like Blue Highway and Seldom Scene. Very bluegrass.
My husband plays a Gibson mastertone, but he also plays a Stelling which puts it to shame. And you should see all the banjo-breaths hover around it, with their Gibsons lying there neglected under the drool.
We've had some great jams when the lonely harmonica or accordion showed up and added a delightful touch to the breaks.
I personally detest the sound of electric bass in a bluegrass band, even T. Michael Coleman. The bass player in IIIrd Tyme Out (Ray Deaton) finally switched from electric to acoustic. Folks in our house can play both (except me, only acoustic) and agree that the sustain is too much for bluegrass on the electric and the sound guys don't seem to turn it down enough.
However, the 5 standard instruments work so well together that they have a magic that is enchanting. Monroe experimented with other instrumentation and settled on this mix, and I'm glad.