I wouldn't trade Brown If I were the Leafs looking in as a Sens fan, but he doesn't hold the value around the league that he may within a year or two should he breakout, that he does for the Leafs right now.
He is similar to the value Stone had for the Sens prior to his breakout in the NHL. We knew that he was worth a lot more than what was perceived, but he was still held out of the upper echelon of prospects (not even sure he appeared higher than 40 on a top prospects ranking). He dominated junior to a bigger extent, and AHL to a very similar/bigger extent. I'd keep Brown for at least a year to determine what he can be at the NHL level, this season should be a big indicator of where he is.
At this point, as a Sens fan, I'd love to get Brown, but not at the cost of a guy like Ceci.
Ceci is a 22 year old, who IMO is the Sens #3 right now. I was never sold on him until the acquisition of Phaneuf. After watching him not have to drag literal dead-weight around (Cowen, Weircioch, Boro) on the second pair, he turned into an absolute machine on the back-end.
It would take a severe over-payment to sway the Sens into trading away Ceci, this is the first time they have had a stable top 4 in nearly 5 years.
Nylander is a gun, a top prospect. I highly doubt the Leafs would even offer him for Ceci, but the Sens still wouldn't make that deal. Ceci means too much to the Sens.
I missed Brown being in the original proposal, he is the best piece in the OP's offer. Dermott and Leipsic do nothing for the Sens, and a 2nd, while valuable, adds nothing towards what the Sens are doing right now.