For really complex, evolving and well just weird polyrhytmic music - it would be awesomely amazing if each track could have individual step lengths! I imagine each track would loop by itself separately from the others in the same pattern.
I’m new to Renoise though, maybe something like this is possible already?
You might want to check out phrases. You can play back different phrases at with different LPBs simultaneously to achieve this effect. 
Oh wow that’s cool, thanks!
Make sure to add another pattern block in the arrangement, and set it to loop, but don’t put any note data in it, just leave it empty.
That way you’ll get the full cycle of all your phrases bouncing off one another without having them re-trigger after the maximum 512 lines.
That sounds absolutely perfect, cheers!