I have some suggestions for types of splitters and mergers that increase in-game QoL and allow for more intricate and cooler system designs.
First, one that has been suggested and I would also like to list:
1) surplus/overflow splitters: allow for output priority and will only output in lower priority if higher priority backs up. Allows for a neater way of saving intermediate steps without impeding with efficiency of the steps after that, or having to create belt systems that take 1/8 of a system's input.
Then some new things (I believe).
2) (Remote) Blocker mergers. Allow one to block a belt (or more belts, remotely). I have systems that take factory output into a one belt bus and then filter all the resources with programmable splitters at the storage. If one resource type backs up, it backs up the whole bus system. Being able to block a resource remotely, instead of running up to it and deleting a belt, seems like a cool addition.
3) Splitter/merger hybrid: two inputs, two outputs. A more elegant way of having a splitter behind a merger (I had some belt systems in which I needed to do that). It could also offer some neat "crossover" mode, in which it allows belts to cross without having to go over or under the other.