The issue I have with these 'What if?' threads is that no two situations are alike. A Rask developing behind a prime Thomas on a team lead by Zdeno Chara as the Bruins is not the same as developing on the hot mess that was the Leafs.
The same could be said for Seguin and the Kessel trade. Seguin was quite sheltered playing behind the likes of Bergeron and Lucic. The Leafs did not have that same benefit. Also, going from being sheltered on the Bruins (and eventually playing for the Dallas Stars), to Toronto where Seguin would have been declared the new savior of the Leafs are two completely different situations. Would Seguin excel in a hockey hot bed like Toronto, while constantly being thrown in the spotlight? Would he handle the pressure of the media whom has the tendency to overblow every minuscule detail? Let's just say the pressure to play in Dallas is a little different than it is in Toronto
. Factor in the difference of personnel, coaching etc. and there is no guarantee that in an alternate world, Seguin on the Leafs would be the PPG player he is now.
As a Leafs fan, these are two trades in particular that many continue to discuss today. The point I'm making (or trying to make) is that there are too many variables to definitively say that X player would be the same player on Y team.