Well there could be some "safety rules" like for fluids or what applies when player is placing items over other items (e.g. splitters on belts are allowed, splitters on underground aren't)

Why? Current blueprint system only allows two state: place it if blueprint doesn't affect anything or "best effort" blueprint with shift+click when it tried to put entities it can. But there are lots of cases when it just doesn't work. Assume we have a 4-line main bus part and we want to ...

Right, thanks. Waiting for a fix then!

After the most recent patch came I'm unable to plan through obstacles when holding shift. I'm not sure if it's intended but I didn't find anything related in changelog so it's probably a bug. I didn't find any way to plan with automatic obstacles removing. Here is a video:
https://youtu.be/RbVyHmqCjQ8

Any devs attention here? I'd really like to have an option to highlight expected belt content. Using combinators or filter inserters looks just awkward and plain wrong. There should be any ingame method to mark belts instead of mimiking it via other means.