The Constitution does state that the Speaker must be a member of the assembly, and changing that is almost certainly not going to happen.

However it does NOT say which party the speaker must represent.  In some similar cases (at least in other Provinces) the Speaker has ended up being a member of one of the opposition parties.  For example in Ontario after the 1977 election the Ontario PC Party won a minority.  The Speaker elected was an NDP member.

I doubt it will happen this time out though as the Liberals have already stated they won't allow any of their members to put their names forward for the position.  The Speaker potentially could be a Green Party MLA, though again that seems unlikely.