From what I heard, Blizzard already has a desired position for you in the ladder, no matter you win your first placement or not.
For me, I ranked up in 2v2 and 3v3 but then I stayed in Diamond for 1v1 :P
So I'm guessing if you did well last season in a league, there is a huge possibility you'd rank up.
I'm not completely sure, because for 2v2 and 3v3 I won, so that might've contributed to my rank up.
Even the smallest donations help keep sc2sea running! All donations go towards helping our site run including our monthly server hosting fees and sc2sea sponsored community tournaments we host. Find out more here.