TL;DRCosmetic only filters that can be applied to belts to act as labels
What ?I'd like to see some way to mark belts with labels. I think this has been suggested before (viewtopic.php?f=6&t=38788), in the form of belt history, but I have a different, simpler suggestion. Simply update the belt UI to include some filter slots. (Like the filter inserter.) If these are set, the belt should show these filters in the alt-view, as well as in the blueprints. 2-4 would be sufficient for most people, if not, you could just label multiple belts. I would suggest that these have *zero* gameplay impact, they would simply be for cosmetic purposes. As such I think any impact on performance or memory usage should be minimal. Since all the basic elements of this are already in the game, I would think it would be pretty easy to implement as well.
Why ?This would allow you to tell at a glance if a belt has unintended contents. If the belt filters also appear on blueprints, it could make many blueprints a lot clearer as well. Any blueprint that requires certain things on certain belts would benefit. (i.e., any assembler setup) If possible, it would be nice for the 2/4 slot-labels to rotate with the belts, so when you rotate a blueprint, the belt labels rotate too. (Some designs require belt to have one thing on one side, and another thing on the other.)
This would also allow some more preplanning options. You could for instance, mark a belt in a bus with a label so you know what you're going to put there, even if you don't have the production yet. You could even put part of your main bus into a blueprint, with proper labels. (If you're into that. )
I made a mockup to show what I have in mind: