Sounds great. There is a little bit of that already on Steemit but not much and def need more specifically geared for hip-hop for sure. Keep me updated if you come up with anything. I am going to start some stuff like that myself in the future after I up my steem experience level a bit.
Just posted a new open mic entry
https://steemit.com/openmic/@elamental/steemit-open-mic-week-72-the-program-an-origional-song-by-elamental
although this open mic is not really geared towards hip-hop.
Also, here is the link for the openmic I entered if you want to check it out
https://steemit.com/openmic/@luzcypher/steemit-open-mic-week-72-record-your-performance-and-win-steem-sponsored-by-pfunk-and-luzcypher