i think both can be done, no need for compromising. the problem with CL is its size/complexity, which is the complete opposite of Arc. also i doubt mzscheme will be Arc's final platform
that said, i've been wondering how quick it would be to grab the source for DrScheme, shoehorn Arc into it, and release it as a single-install, works-right-outta-the-box IDE
Using mzscheme as some sort of high-level virtual machine isn't a bad idea as it may seem. In fact, I think that it could be quite practical even in the lon period. Speed is not blazingly fast, but it is quite good, and you get stability and a good portability without effort.