What are you looking for in day packs ? Is it a stand alone day pack, or a day oriented pack to be used on the same suspension ? What would the perfect day pack on the suspension look like ? What features as far as CI, and pocket / org capability would be desired ? In my opinion, volume should be somewhere in the 2000 - 2500 CI range. You need a place to store some essentials, and then potentially optics or tripod as well as insulation type gear.
Please chime in, this is your opportunity to have feature requests.