I don't know how terribly pertinent my input is, but I do have a fair interest in this topic.
My background: I am one of the GDMO's in Edmonton. Been in the uniform for a year and a bit, no overseas taskings/tours as of yet. I have been issued the STOMP II and....as you all have mentioned, I find it to be more of a useless CRAP glutton than a streamlined extension of my SKILLZ

. I am also an avid backpacker and so have some wisdom to gather from there, if not from tour experience.
So, first, a disclaimer.....my task as an MO is probably going to be drastically different from anyone taking a field medic role. My Medical Bag is basically to allow me to stock a facility while its getting established. Be that a UMS, BMS; or possibly a helicopter during field exercise. Once the facility is established for its current tasking, my need for a Medical Bag should become almost irrelevant.
But, to get on with my thoughts.
Yes, the ideal bag would be modular, at different levels of the complexity of the thing.
First, it must integrate both medical space and "living space". The living space probably would go best on the top of the bag (for reasons I will point out shortly). So, it probably should take on a general concept of the 64 Jump Bag to begin with. True, if you were to make the "universal bag" it would probably be too large....so we have to make some assumptions. Looking at the combat intensive missions that have been executed over the last x number of years, it would seem in my unresearched opinion, that we tend towards warmer climates (at least not the arctic). Coming up with a standard "personal kit" list that reflected that would put you at a starting load size for the "living space" compartment. Divide this into a Pack Body and a Valise to keep things familiar and include a number of attachment points to allow for external pockets to be firmly attached to allow medics to include some extras they need to keep sanity (or to attach a "cold weather" pouch to include more compressed clothes). Take some modern approaches to load distribution and compression methods to turn this into one solid singularly-moving unit.
Now for the Medical Space of the system. It should be to the bottom, and be rigged in such a way that it can be snuggly attached to the bottom of the "living space", can be dropped right off the pack altogether, or can swing around to the front of the medic so they can access items from it without necessarily taking the pack off, and then can be carried in the front (as a waist bag or chest bag?!?!), or can be swung back behind.
Now, the materials for the pack should be "military" tough, but should be pretty light-weight....afterall....there is alot of stuff to carry already. Moving towards a medium weight silicone impregnated ripstop nylon would be good for both main parts of the system.
As for the internal modularization of the "medical space", I would suggest moving to a silicone impregnated 1.1 oz ripstop nylon....incredibly tough for its weight and see through...which will help locate items quickly.
As for what modules to include, what sizes and how much variability....that now needs some sort of research to find out the frequency of injuries/illnesses encountered, weighted by their relative severity/importance. Then, you can figure out the likelihood that a particular injury/illness will occur over the length of a tour/daily tasking/expiry duration for medical supplies. Once you come up with your short list of what needs to be treated, then you can create your equipment/supplies lists....organize them into categories and then design modules to meet the needs of the categories. Eg: the airway module would have a oral-pharyngeal airways, nasopharyngeal airways, laryngoscope, proper blades and backup batteries, ETT tubes of appropriate sizes, bougies, stylets, lidocaine spay, limited number of syringes and sedation drugs, as well as some type of portable/collapsible bag-valve-mask assembly, and a compacted trach kit. At least that is what my airway module would look like (and I am a doc, not a medic so my wants are probably quite different). Then, within the module, you design the order/spacing of the items so that it can be maximally compressed with straps directly on the outside shell of the module.
From there, you then need to design the most logical way to organize the different modules (eg airway module easy to get to, and near to the ACLS module, but not in the way of what might be considered a TCCC module; the TCCC module items might best be served by being in external pockets on the "medical space" subsystem of the whole bag system.
But, I could ramble all day....
Maybe, if this is considered a good direction to go....
It might be worthwhile fueling a discussion of the items different medics carry in their current bags, what items they wish they could get in there and what conditions they find they treat most often/most importantly. From there the modular categorization might begin.
Well thats what I got to contribute.
Dr. Cyanide