You know I'm definitely looking forward to the Ultimate Armies book. Been waiting on that one for a loooooooong time, since it is required for the work I am trying to do and jury-rigging the current armies with different races and different layouts has so proven difficult for me. If you did an Ultimate Marketplace book (a book about trade goods as I have only found one book on trade goods), that would be great. Someone above mentioned Ultimate Strongholds and I agree entirely on that one.
On second place, Kingdom-Building would also be great.
Third place would be... well, those are really the only two I have interest in, really. So, whatever else that floats your boat.