Midi Note Length Manipulation Tool (patcher , fl studio)

I find a lot of producers, know which notes they want played, but often lack rhythm. (They either can play individual notes on piano or program it with a third party tool like cthulhu)

Is there a tool that takes individual input midi note, and does note length manipulation (either lengthening notes, or shortening, or leaving rests), spits out notes played with some sort of structured note length. (I picture the note lengths being driven from a study of note lengths in different songs, and a sort of different presets built for note lengths, or some sort of random generator for note lengths until you get what you like.)

Does such a tool exist, if not, could it be built in JUCE, and if so, how would one go about building it?

Specifically looking to use it in FL Studio Patcher,
MIDI => Cthulhu => Note Length Manipulation Tool VST => Serum => Audio Out

Cthulhu in this case, would feed notes into the length manipulation tool, to which the note manipulation tool would change the lengths and add rests between notes as seen fit.