I wouldn't make any short term moves for real value, I'd be looking at a long term PF or guys basically being given away for the bench. I wouldn't give up a real asset for an older (i.e. not long term) bench player. So guys like Richardson I'd take back, but not at the expense of Nesmith or a 1st. A 2nd or two and cap relief, sure.
As I've said elsewhere, Wood would be my main target. I think he'd be a great long term next to Rob where they could play together or as the center without the other. Offensively, does what Horford does. Isn't the defender, but athletic and long enough I think he'd be ok at it.
I'd try something like Wood for Nesmith, 23 1st, 25 1st (lotto). See if that could get it done. I'd probably do more if needed, but there isn't a whole lot more Boston can actually offer.