I think he is very tradeable, and I think we can be very flexible with what we can back in return.
I don't think we necessarily need to shoot for expiring contracts in return. This is most likely a multi year rebuilding process for us so I don't think cap space is that important for us next summer. I don't think if we traded for an expiring in a Bass deal we would have enough cap room to be major players anyway.
If we are patient when on 12/15 teams can then combine players that were signed over the summer through the trade deadline, we could possibly have a few more options especially if some injuries pile up around the league.
Another thing to consider is what is Bass's competition from other bad teams trying to offload a PF with a similar size contract and skillset. If Orlando falls apart BBD might be on the market. Same with Channing Frye, Thaddeous Young, Amir Johnson, JJ Hickson, and Chuck Hayes. However right now Bass is probably the top player from this group along with Young.
I do like a couple ideas people have mentioned like the Asik idea. The other is Bass for Ariza, however we are taking more money back in that trade that I think pushes us the luxury tax. Maybe someone like Bass & Brooks for Ariza and a protected pick. However Ariza is playing so well for them right now, and Porter isn't playing. Maybe put this in our back pocket for now.
A trade I just tried in the NBA trade machine is Stoudemire for Humphries, Bass, and Lee. This helps NY a ton for not only being competitive the next couple years, but also freeing up cap space for next season. Since this is an unbalanced trade in NY's favor with Stoudemire basically almost finished, and I realize NY doesn't have any picks to trade until 2018, I would ask for that 2018 pick unprotected, maybe the right to swap picks in 2019, and see if we can squeeze another pick out of them. That would cover us for about 6 years with chances to land lottery picks.

Another one I came up with is Bass to SA for Bonner, Joseph, and Baynes. Basically dead weight for SA, and improves their team with Duncan on his last legs. We should at least get a protected future pick out of this deal.