You should probably spend some time browsing through Lambda the Ultimate [1], a discussion forum devoted to this kind of thing (it is very academic though).

There's a collectively authored set of guidelines "How to Design (Declarative) Programming Languages" [2] posted in comp.lang.lightweight, curated by Scott McKay that you might find useful.

[1] http://lambda-the-ultimate.org

[2] http://thread.gmane.org/gmane.comp.lang.lightweight/2547/foc...