I'm not sure there is any benefit to #1 and #5.
For 1, the build won't be any cleaner because the merger/splitter has to make room for input/outputs. That would simply amount to exactly the same design layout as leaving it as it is now and just connecting a conveyor belt with a 90 degree turn.
In fact, I think that it would make builds harder and much less clean. The current mergers and splitters are simple boxes. It's very easy to squeeze them into small spots. However, if you have all of the outputs/inputs facing the same direction, then the shape of the merger/splitter needs to increased and elongated. This would make them much harder to place and consequently make them much harder to interact with.
In fact, I have a specific example in my build where I used 2 mergers, a splitter and a smart splitter to filter an assortment of oil products into a truck station. Basically, I wanted to sort fuel out of the line and put it into the fuel input of the truck station. But I only wanted it to do that while the station had room for fuel. I ran into an issue where the entire system would get clogged once the station filled with fuel because all of the fuel was trying to be split off the line, but the output was full. Therefore it held up the items behind it. The solution was to put a second splitter after the smart splitter that would then merge every 2nd fuel back onto the filling line. So, now once the truck station fills, all the fuel simply goes back onto the filling line. This would have been impossible build if all of the inputs and outputs where in the same direction because of the cramped space I built it in.
Now, it might be better for specific builds where you want all the inputs/outputs coming/going from one location, but for builds where they AREN'T coming/going from one direction, it becomes far more difficult.
As for #5, that also doesn't really make much sense to me because you can just build a chain of mergers/splitters to reach the desired number of inputs/outputs.
For a 1 to 2, just don't connect one belt.
We already have 1 to 3.
For 1 to 4, daisy chain two together, and leave one belt disconnected on the second. Connect it for 1 to 5.
Repeat the 1 to 4 method for 1 to 6.
The way that you would construct this would take up exactly the same amount of space as a specific 1 to X merger/splitter.
Everything else I agree with. For #4, I'd phrase it to be a ratio. Like for every 1 item going left, I want 2 going right. This should be an option on the smart splitter.
