The problem is that the type of player (in contract, production, and age) you're asking for is the type of player we need to add as well. Giving up Kells is counter productive to the goals and time line of the org.
I'm not saying we wouldn't trade him, but it's unlikely. It's like someone asking for Tkuchuck, he checks all your boxes and you don't have an excess of that player currently, it'd take a large sum to pry him away from Ottawa, and likely changes the time lines Ottawa has set.
Chychrun was a similar contract for us, except he asked out when management approached him regarding the subject. The return for Chychrun (who has a few warts, namely health) was solid, and I believe Keller would be worth equal, if not more than 12th OA + 2x 2nds.
Taking all of the factors into consideration, I'd ask for a pick in the 10-15 range, plus 2x picks in the 25-32 range minimum. No idea if someone would pay that, but it's irrelevant because I'd either keep a player I really like in Keller, or get value I deem acceptable. Both are wins.
Now add in Joseph and Brannstrom, players I assume we don't want, that will take up 2 roster spots as well as salary, and you need to compensate us for that. That's what, a 2nd? Maybe more? You might be better off finding homes for them seperate of this deal as you could maybe get value rather than lose value on them.
Just my thoughts, though probably wrong.