Depending on your needs you might look at the Medic bags and Packs from London Bridge Trading Company (http://lbtinc)(no affiliation). I used one of their training coverage bags for quite a while (it is now being used by an MD for International Rescue work). Pricy, but of super quality. Not waterproof (although I think they made one that way at one time) but very rain resistant. Lots of loops and mesh pockets for your quick necessities. Larger pockets for removable "purpose" bags like an airway kit.

There are also others who make similar items like Tactical Tailor, but I have not worked with them myself.

Respectfully,

Jerry