I'm certainly not against the idea of a Severnside airport in theory although the practical aspects might be more challenging, as well as political and nationalistic sensibilities relating to the actual location.
The ideas/schemes put forward in the past have usually featured an airport on the Welsh side of the estuary although a Pilning (north of Bristol) airport was considered and rejected in the 2003 government white paper on the future of civil aviation.
Some have linked the much discussed Severn Barrage idea with a Severnside airport enabling road and rail traffic to cross the estuary on the barrage. The Barrage seems to have more support in Wales than it does on the English side of the river, not least because it would emasculate Bristol's port facilities.
Had earlier generations exercised a greater degree of prescience and co-operation (although in fairness few if any people prior to this century could have seriously predicted how busy airports such as Bristol would become compared to their previous somnolent existence decade after decade) Bristol Airport might now be FZO/EGTG instead of BRS/EGGD.