It sounds like your well into your design phase, but I just read your post and it got em thinking... I use a traditional 3 compartment (2 insulated, 1 not) bag that I usually stuff with 2-3 rackets for play, and when it's in my garage it holds all 6 of my racquets. It has a shoe compartment which I never use because it's impossible to clean, poorly ventilated (or I'd probably store my tennis shoes in there). There is also an outside compartment for cell phone / i pod / etc. I have a couple towels, a tape measure (for checking net height), and some overgrip stuffed in there.
Basically the bag is my locker, and when I need to take it to play, I either end up grabbing a racquet out of it and go hit, or dump all the extra stuff out and take my gear if it's something where I'm likely to want more than just my racquet and the shoes on my feet, which is actually kinda rare for me. I'd say a lot of the guys I play with are like that -- show up with racquet in hand -- and way more of the women I play with actually bring the giant bag with them to the court, even through they also just play with one racquet.
I think the modular idea is great, so you can lock everything into your bag and basically have the complete locker set up for keeping everything in one place in the garage, but then detach the "minimalist player" part to grab and go.
In that vein, the most efficient bag I've used was actually for my daughter when she first started playing. It wasn't even a tennis bag - just a drawstring top canvas back pack (see example pic 1 A). It cost me < $15. I've seen one woman carry her gear in a similar setup using a fancy leather purse with the drawstring top to carry two racquets, tennis shoes and what a change of clothes (see pic 1B). I think leather or canvas gives enough padding to keep the racquet from getting chipped, but I wouldn't use nylon, which is what most of the drawstring equipment bags are made of (pic 1C). I've seen a lot of zipped racquet backpacks where the racquet sits on the outside (see example pic 2). That seems like a pain to get at the stuff that's in the backpack, and a good way to crack the yoke. I also bought her a typical racquet backpack (example 3) which she never uses because it only hold one racquet and she wants a backup racquet at every match.
Now I consider myself something of a bag nerd, by virtue of having bought what feels like a warehouse full of equipment bags for my kids various activities -- soccer bags, swim bags, softball bags, hockey bags, ski bags (both boot bags and board bags -- see example 4) and of course tennis bags.
I think a really neat design that I've never seen would be to make a mega-size 12 racquet bag on skate wheels, like a hockey bag or a snowboard bag, and have a sling strap also (because it should still be "light" enough to carry. Insulated compartments, separate shoe compartment with lots of ventilation, and then attached to the front of it, one of those 2-racquet grab and go rucksack bags that is also a backpack. That rucksack bag should hold 2-3 racquets, space to toss towels, overgrip and other tennis provisions in the main compartment, and also have an outside sleeve pocket to hold cell-phone, ear-buds, a few papers, etc. Also a handle on top to grip it by when lifting off the ground, and an elastic mesh side sleeve to hold a water bottle.
example 1A (like this,but with a drawstring top):
example 1B (not a tennis bag, but can be used as one - stands up on it's own nicely):
exmaple 1C (the cost cutting answer to racquet covers - include a "bag"):
example 2 (what is protecting the yoke of this racquet?):
example 3 (this could be attached as a module to a bigger bag like example 4):
example 4 (a good "locker" bag setup - shoes on top, insulated racquet sleeves accessed through the side, and the rest of the main area for towels, grip tape, clothes, etc ):