This is probably a repeat thread, but what the hey.
The guys behind Homstarrunner have their own lineup of humorous games, including their own sierra-esque adventure title Peasants Quest. But i'd like to see an adventure game involving the actual cast of Homestarrunner.com, perhaps with Strongbad, Strongsad and Homestar as playable characters. The casual, non-vital dialog might be worth the price of admission.
Calvin & Hobbes
Between summer + winter activities, school, and the power of Calvin's imagination, this would make one hell of an adventure game if created with respect. Just imagine the character animation retaining the expressiveness and motion of the original art. Too bad Bill Waterson would never allow it.
Anything referencing the late Zdzisław Beksiński's work in the overall art style
While a style isn't an IP, just look at this man's insane genius: http://bsd.miki.eu.org/gallery/Beksinski?page=1
I'd be both thrilled and scared to play an adventure game looking like that.
So what intellectual properties would you like to see in a point & click adventure? Or designers for that matter?