I think you will find that a bag of about 4000 cu in capacity will give you maximum versatility - cinch it up for day trips and expand it for longer jaunts. You can use a bag of this size for trips of up to a week quite comfortably. For true expeditions, I fear there is no recourse other than a dedicated pack of upwards of 6000 cu in.

Modules make a lot of sense, but I would think in terms of units that can be placed inside your main sack, rather than exterior units. You do not want a lot of stuff on the outside of your pack, unless you are traveling exclusively on well manicured trails, and never putting your pack in boats or planes.

I would agree with Teslinhiker that a well designed mod system would be very nice. That a good one isn't readily available says something.

For longer trips of any complexity, I think you will find that you will be packing unique configurations of gear in any event.