May I suggest that the building opposite the President's Palace in Valletta, known as the Main Guard, should house the Parliament building.
Though there may be some difficulties in re-locating the restaurant and the Italian Cultural Institute, I presume that the government will have the ways and means to overcome this hurdle.
The back of this building is partly on Strait Street where vacant building could be acquired for additional space.
The proposed underground parking planned under the square will be one big holistic project that would have the consensus of everyone.
Please do not put Parliament in a former opera house - the saying is too rough to pronounce in the press.