This suggestion requires the implementation of a new item OR a change in function of an existing item, as well as a popular suggestion in the from of crouching:
I'd like a way to stand still on, or run against the direction of belts WITHOUT just narrowly balancing on the edge. I often build belts over gaps or around mountains, and at MK1, it's easy enough to use those same belts as a bridge. At higher levels, using those belts can be a bit harder. Being able to stop on a belt at will can serve to provide a look at production, throughput, belt networks etc from angles and view points that are useful but otherwise unavailable.
So my suggestion is the Mk2 Blade Runners, or "Belt Runners." While equipped, a player's speed is not influenced by the speed or direction of the belt. A player can step on a Mk4+ belt without being hurled across the map against their will, and can stop easily on a moving belt to work on something that otherwise doesn't have easy access. Riding the belts and being thrown halfway across the map is sometimes considered a feature, so a player can resume riding a belt as normal by crouching, which re-enables belt influence on player movement. An alternative, without crouching functionality, is that belt influence would be negated while holding down shift, and reinstated when shift is released. Making it optional at higher levels more firmly defines it as a feature.
Crouching functionality has already been requested, and currently stands as a suggestion with 240+ upvotes on this suggestion board. Aside from the exploration feature of ducking behind a rock to dodge a fireball, it's largely suggested so that players can ride belts through windows in their factory walls. That functionality shares some purpose with the Belt Runners: Surveying your production line, or quickly getting to a spot you might not normally have easy access to without otherwise disassembling your factory. Many, many building have maintenance ducts. (Maybe holding shift while crouching would let a player with belt runners move back through the window against high-speed belts).
The model doesn't have to change, but if you decide to add something that visually explains the functionality, maybe stick roller coaster wheels/rail casters on the ankles, implying that they lock in to the sides of the belts. Bonus points for emulating Twilight Princess's Spinner and including the following functionalities: If players "snap" into a belt and have to "jump" to disengage; if player movement on belts once "snapped in" is faster in both directions than normal running; and if the mk 2 blade runners can snap to and improve running speed on train tracks, too.