A & B mixing channels with a crossfade.
Monitor channel to set up you work before displaying it on the main output. Sorry, realise this is major, but it has to be important, unless you have a better method planned? I like the simple auto-fades very much, they are going to be really handy for bring in the new material. But if this is intended as a stand alone performance utility, monitoring work really has to be a part of it.