I don't think Collison is trash. He's playing half of his minutes at SG this season which is a terrible way to use him, but that point aside, I have been considering a similar kind of deal with Detroit. Brandon Jennings is wasted on the bench, but at this point in his career I don't see a starting spot opening up for him anywhere. Stan the Man loves shooters and needs more backcourt production so Collison and Belinelli should theoretically be desirable targets. Jennings is more of a Rondo style PG in that he'll run the offense and try to set guys up. That could result in a smoother transition between starting lineup and bench play. He is an erratic shooter though and losing Collison's efficient scoring would hurt.
That's why I don't think Jennings is the key target. He'd be a Collison replacement (since it's awfully hard to sell somebody on Belinelli alone as a deadeye shooter when he's currently at 31% on the season). Assuming the deal has to be Collison/Belinelli for Jennings/? and we're downgrading a little at backup PG, my other target would be (surprise, surprise) Stanley Johnson. We can fill Marco's spot in the rotation with a combination of guys we already have: Ben, Seth, and Omri. That opens up more minutes at backup SF and what we need even more than another shooter is a perimeter defender. Johnson isn't lighting the league on fire, but he's had his moments. The rook would actually be an upgrade from the three point line at their current percentages and he's young and improving and, more importantly, a hell of a lot better on perimeter D than Marco is.
Salaries match: (Marco 6.3 + Darren 5) = (Brandon 8.3 + Stanley 2.8)
And an we're not giving up any key pieces, so hopefully this doesn't have a negative effect on team chemistry. Darren is an important part of the team but Jennings fills most of his production in the short-term and we only have 1 more year of Darren on his current contract anyway. I don't think we need to make a big trade, just a sensible swapping of mismatched pieces and this is a deal which could benefit both teams.