I think there are two keys design points that should be kept in mind:
- Allow people to play according to their playstyle. Factorio has always been a game you can play in many ways, and having an option (such as logistics bots), doesn't force anyone to use them.
- Instead of just tweaking numbers to make one playstyle more tedious (because that's really all increasing costs will do anyways), provide methods of making the other playstyles more fun.
- Tedious upgrades, transitioning between tiers of belts is extremely tedious, with basically no automated way of doing so in vanilla.
- The limited range on beacons make high-throughput builds with belts rather annoying, there are several ways this could be changed, such as a non-square transmission area (like 8x3, though graphically that doesn't make sense), capping effect bonuses, etc.
- Mediocre throughput, even the highest tier of belt just doesn't move many items/second, and you need many, many, many lanes to get to a serious throughput, which again amplifies the problem of tedious upgrades, and usability with module+beacon builds. Although an extra tier (maybe 30 items/sec/lane) would be nice, I think the idea of stacked belts have much more potential. Having two layers of belt (or effectively 4 lanes), and maybe even something like a vertical splitter would imo be amazing for making creative designs. That'd literally add an extra dimension.