As it stands T1 is getting completely demolished (save for pier B). That creates a huge amount of space to eventually increase airside facilities if the need arises.
I have seen plans for an expanded T3, which predominately involves an extended pier to the north east (I hope that is the right direction!) with the car park being built upon to provide additional stands.
That may be a solution, but those plans (I think) pre date the MAN-TP and I would be surprised if MAN did not have an ambition to also demolish the T3 site and construct a new, larger terminal somewhere around where the existing roundabouts/crowne plaza is now situated. This would entail a huge amount of demolition but would provide expanded airside facilities (including the possibility of more finger piers) as well as a possible double taxiway for runway 1. It would also being the terminal much closer to the existing classic rail station and closer to the new HS2 station.
Clearly a long way in the future, but food for thought.