We can only use Fournier in a trade for Siakam if he agrees. The only way I see him agreeing to a trade to Toronto is if they over-pay and I don't see why they would do that while giving up Siakam. I would love to get Siakam but I don't see it. You would have to put Brown on the table, then Toronto may listen.
So trade Brown for Siakam and sign Fournier ourselves and you have:
Smart
Fournier
Tatum
Siakam
Horford
Richardson, Williams, Parker, Nesmith, Langford, etc.
Not horrible but I doubt anyone wants to give up Brown.