I second the request for Alley Oop, which also seems like it would fit perfectly with the series they're already releasing (i.e. adventure strips, unlike Fantagraphics which tends to focus on humor strips).  Those Kitchen Sink collections are incredibly good.  Another one that would fit well with the other strain of LOAC, latter-day humor strips such as Bloom County and Family Circus, is Doonesbury.  There are plenty of Doonesbury collections, but they skip around half the material.  I'd love to see either of these!