Ideas that are too old (too many things have changed since) and ones which won't be implemented for certain reasons or if there are obviously better suggestions.
Nope, i do not like that idea. Readability would be terrible so it would require a underground gfx redesign. Lane split made on angled underground belt input would be possible on straight belt and would be switching lanes. Item positions (including the invisible part) would not be continuous and building another underground belt in classic straight continuation would be non intuitive. It would be making a spaghetti even tighter and more obscure.
There's always something that could be theoretically be "slightly smaller". Three-lane-splitters, belts with opposing direction lanes, etcpp. Can't have it all. Build larger instead of smaller :D.