TL;DR
A device that can receive belt feed of any stack size and output stacks of a set size.What?
Currently, there's no simple way to un-stack stacked items on a conveyor aside from having inserters transfer them from one conveyor to another. While some have suggested an option to limit the stack output size of Stack Inserters, Big Mining Drills and Recyclers, but this is only a band-aid solution imho, the player would need to manually set it or disable it for each one they place. There's also no way to stack items aside from the Stack Inserter, which is fine, but I feel limits what can be done with the stacking mechanic in terms of improving factory throughput. I also find it frustrating that stacks do not play well with mechanics like, say, belt balancer layouts and belts that are fed from multiple sources, because if you have a combination of Stack Inserters and non-stack Inserters feeding a belt in any way, eventually you'll end up with a mix of stack sizes as the stacks are reduced when machines take items off from them.The Stack Leveler, on the other hand, could be a simple way to even everything out. As I envision it, when a Stack Leveler is placed on a belt feed, it automatically receives and outputs items as quickly as possible, that is to say as fast as the belts it connects to run into it. The Leveler can be set by the player to a particular stack size, and the output will be that stack size.
For example, say you have a Red Belt of Iron Plates stacked 4 high feeding into a Stack Leveler that in turn is feeding a Blue Belt. You can set the Stack Leveler to output stacks only 2 high, and that Blue Belt will be fed a steam of Iron Plates 2 stacks high. In this hypothetical scenario, the Blue Belt is not fast enough to carry the output at the maximum speed the Leveler produces it, assuming the Red Belt is full; 30 stacks of 4-high Iron Plates directly converts into 60 stacks of 2-high Iron Plates. So you'd have a full Blue Belt of 2-stack high, but the Red Belt would not flow at full speed. Conversely, if you had a Yellow Belt if Iron Plates feeding the Stack Leveler, that's 15 stacks of 4-high Iron Plates, which converts into 30 stacks 2-high, a less-than-full Blue Belt.
However, this is where synergy between stacks and belt speeds comes into play, because there are four speeds of belt that divide into each other, and stacks go up to four. Thus with the Stack Leveler, the player can optimize throughput with a bit of math. As another example of how this could be done, say you have a Green Belt of Copper Plates that are not stacked, they're running at 60 items a second. Feed them into a Stack Leveler outputting stack height of 4, and you can have a full Yellow Belt of 4-high stacks of Copper Plates coming out. Then later down the line you could feed them into a Splitter with two Stack Levelers on the output side and get two full Red Belts of 2-high stacks. Or you can reverse it, and have a fully stacked Green Belt feed four Red Belts via a Splitter-Leveler combination so you get four Red Belts of 2-high stacks. And since each Leveler could be set individually, you could use this to mix up the outputs, like in this case having two Red Belts of 2-high stacks and two Yellow Belts of 4-high stacks.