Three factors: where would Andre want to go, where would the Texans not want to trade him, and how much salary cap space does the team getting him have.
To me that leaves two teams: Seattle and Philly. Both have the cap space, both are outside of the AFC, and both have teams that could contend for a superbowl. However, because Philly's QB situation is a bit more muddled and because Seattle has shown that they can get to the Superbowl and because Seattle is desperate for WR help, I think Seattle is the likely landing spot.
Indy would be a possibility if they were not in the Texans division. They have the cap space, a great QB, and could use a compliment to T.Y. Hilton. However, the Texans worst nightmare is having Johnson succeed in Indy, especially in games against the Texans.
New England seems like a possibility but they are currently $12 million over the cap, so unless they can free up significant cap space, they cannot afford Johnson.
The Cowboys also do not appear to have the cap space to get Johnson.
I think any team trading for him is likely going to demand a restructured contract. Right now, he is set to make $10.5 million this year and $11 million next year in base salary. The Texans absorb all of the deferred money on his contract when he is traded. I could see a team paying him somewhere between $6 to $8 million for each of the next two years, guaranteed. Fitzgerald basically got $22 million total guaranteed over the next two years, so Johnson getting between $12 and $16 million over the next two years sounds about right.
By the way, I think whatever team gets him is getting a great player. Given the right situation of QB and offense, I think his numbers will far exceed what he did this year. He has not really declined over the past two years, rather his QB's have been horrible. He may be a little bit slower, but he still runs great routes and still can catch the ball.