The whole pattern matrix needs to be reworked to allow different pattern lengths and pattern numbers per track…
Pattern numbers per track: by using slot aliases (ALT+Drag in matrix), you can combine different patterns in the matrix
Different pattern lengths: you suggested this elsewhere: https://forum.renoise.com/t/different-pattern-lengths-per-track/42088
There also needs to be some type of pattern pool to view and edit patterns without having to put them into the arranger sequence.
Something like this, perhaps?
https://forum.renoise.com/t/free-roaming-through-patterns/40209