Honestly, this is what would catch my attention for 4e...
A big book of alternate Paragon Paths, powers and builds for the classes available in the PHB 1. I think if someone produced a big book like this, with quality material and at a fraction of WotC's 3 books@ $34.95 price tag...I'd be all over it. IMHO the core PHB is severely lacking in options when it comes to differentiating characters in the same class, and I think the above would help to flesh out the core for those who don't want to jump on the supplement train.
Next, would be a cool campaign setting. Honestly, I think WotC may have dropped the ball here with the FR being drastically changed and released in two books. Personally I have no interest in the FR...I'm not keen on generic high fantasy unless it does something original. Now I would have been all over Eberron if it had been released this year (actually I wished they had done this and seen which one sold the most...then supported that one with books and the other in DDI), but it wasn't...so now is the prime opportunity for a company to create a campaign setting that will grab those who aren't interested in generic high fantasy.
Finally, Monster books...unless you do them like WotC's MM. Make a MM that someone really wants to sit down and read, that also inspires adventure ideas and the imagination. Don't do a book of pure stats with pictures.