Welcome to the FOG and Festool. This topic is one that I'm always indebted to other people for their amazing ideas with all things Tanos or 'Systainer World'.
Like you and everyone else, I buy tools that are already in Systainers, I buy their accessories which are often also in Systainers. I buy Abrasive Systainers to support my sanders. Sometimes I switch another manufacturers tool kit/container out for a Systainer that has the option of a better insert /storage usage than what it came with originally like my Fein Multi Master..But for small items or the odd group of something, I tend to study what others here on FOG have done and shared with all of us.
Very creative group, very global...
I bought a Sortainer as a First Aid Systainer. The drawers are easy to open if you've got a cut or a splinter on a finger. At the time, T-Locs weren't an option.
All the medical supplies fit in their divided sections really well.
You can do the same thing with screw assortments or other fasteners, you just figure out your needs and buy the right drawer unit or regular Systainer after that.
Don't forget about making your own plywood or metal inserts for a bare T-Loc to gain something that you can't see pulling off with a Sortainer.
Lastly, people disassemble their Systainers and add sections to them or change the lid to create a different version because they wanted the special lid on a larger/smaller Systainer. Wasn't sure if you were aware that that's an option with the Tanos System