A company called BrightLine Bags has a compelling offering: Gear bags that consist of modular sections that zipper together, allowing you to configure the bag as you like.
Center sections are offered in sizes ranging from 2 inches deep to 11 inches deep. You can attach as many of these together as you'd like, choosing which one you'd like to attach the handle to. You can then add a variety of front and rear "caps" and side pockets, and internal organizers are offered as well.
The use cases look pretty compelling:
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If you can't decide which sections you need, they also sell preconfigured bags to get you started.