An upgrade in goal is something that can be looked at but it's not a weak spot. Migs is very good but we could get better. Right now though we have far more pressing concerns elsewhere in the squad so Mignolet is safe for now. Im happy with him, his last half of the season was very good.
As for formations. What about the old classic 4-2-3-1
------------Mignolet
Clyne--Skrtel--Sakho--Moreno
--------Can---Henderson------
Milner-----Firmino-----Coutino
-----------Sturridge------------
Bogdan, Lovren, Flanagan, Lucas, Lallana, Ibe, Bacca
Squad depth, Toure, Markovic, Allen, Origi, Ings, Enrique, Ilori, Ward.
Bacca and Clyne I have named as names we have been linked to in positions we desperately need. So instead of just putting New RB and new Striker. I have just put names down to pad out the squad. Clyne looks close. No idea on Bacca though.