After #2443 is done, it would be good to support sos format (same as tf format, but broken up into second-order sections).

I started working on this and then found #1175 which does this, as well as several other things, but is based on Octave GPL code, so can't be included in BSD SciPy. https://gist.github.com/endolith/4525003

Either those functions can be re-written from scratch as BSD, or we can possibly get permission to relicense it. I contacted the original author a while ago and he is ok with relicensing Octave signal stuff for SciPy as long as the people who run Octave are ok with it, but I never got a reply from them. So either follow up on that or rewrite them from scratch.