I would enjoy seeing Mr. Slave more, he was very funny in episodes like "The Death Camp of Tolerance" and "Stupid Spoiled Whore Video Playset". While he's still around now, I think all of his appearances have been significant in some way, and definitely funny!

I actually enjoyed Mr. Slave’s relationship with Mr. Garrison strangely enough, so I wouldn’t mind seeing more of him in the series.

Of course, Mr. Slave and Big Gay Al need a episode focused in theyr relationship (adopting a baby?)