A GUI would certainly be nice, if it can be done in a cost effective manner. I'm sure it would bring your tool more into the mainstream.
However, it seems that a quick win would certainly be to provide an XSD that makes use of the current CAML XSD in the file I referenced. It would include your schema for lists, joins, etc. and reference the CAML XSD for everything inside the <Where>.
Regarding a SQL Query syntax, that could be useful, but for those of us that know the that CAML is used under the covers CAML would probably be preferred. A SQL Query syntax makes sense from an adoption standpoint, but it has to be a limited SQL syntax which could be frustrating/confusing for some.
So, my input would out of these to focus first on XSD (quick win), then on a GUI, then on SQL Query syntax.
Thanks for giving us a forum to provide input.
Kirk
www.threewill.com