A welcome addition would be Mk2 splitters and mergers, where you could set input and output priorities.
Mergers with input priorities - instead of taking an equal amount of items from the connected inputs, have a configuration window (just like with constructors etc.) where you can set up which input belt should be prioritized.
Splitters with output priorities - instead of an output of 1/2 or 1/3 (depending on number of connections made), have a configuration window (just like with constructors etc.) with options to prioritize a certain side of the splitter