I was actually working on a proposal like this and found that @rbanffy had started this one. As an IBMer I'm very frustrated that we have a lot of academic information (manuals galore) but very limited applied knowledge available in the eco-system. My desire for this proposal is that it allows for all manner of questions from scaling, capacity planning, sysplex administration, installation howtos, etc. so that people that are working on the mainframe (in my parlance at least that is generally z/OS but should include VSE, TPF, z/VM, ...) can get and give practical howtos and less of a RTFM which on z/OS at least could take years.
Having been a practitioner for several years before joining IBM (pre-dating StackExchange Community) I felt that this kind of practical approach was something that would benfit those in the field, and, perhaps more importantly in some respects, demystify the mainframe so others can take advantage of all the assets that reside on these mysterious boxes :)