Barrhaven, Kanata and even Orleans could all be well served by OTrain/SBahn-type rail with minimal new infrastructure, not to mention the north of Hull, Chelsea, Buckingham, Masson-Angers and beyond on the Gatineau side. Regional rail to surrounding towns like Smith Falls, Casselman and Arnprior would be more than possible as well: if there's enough demand for an expressway costing hundreds of millions of dollars, surely there must be enough demand to warrant rail service costing a fraction of that.
The lines could terminate at Bayview (if the tracks from Confederation to Bayview are twinned) or Chaudière (Hull), both closer to downtown. It could be implemented quickly, painlessly and would provide much-needed relief to the crowded and uncomfortable buses to the outer suburbs, the congested roads and our high infrastructure costs.
As for Union, I don't think that it's outside the realm of possibility. It would take a near-perfect alignment of the stars, but the project is by no means impossible: from Hurdman Park, make a bridge over the Rideau River where the Nicholas extension is planned to go, then make a shallow cut and cover tunnel under the Nicholas Expressway, to join up with an underground concourse (Ã la Britomart Station, Auckland's new/revived central station) beside the Canal, beneath Union.
Britomart Station in Auckland was abandoned for over eighty years before they rebuilt it underground, much like could happen with Union
If/When we start seriously discussing HSR as a national megaproject and someone in Ottawa champions the idea of a revived Union Station, I don't think that it would be too far-fetched a possibility. If by then there are already O-Train/Regional lines up and running, the project would be very welcome to suburban dwellers, providing them with a transfer-less ride downtown. Downtown businesses and travelers would benefit from a direct rapid connection to Montreal, Toronto, New York and beyond.
I'd estimate the cost at $1-1.5B (Britomart only cost about $200M), which though not insignificant, is definitely within the realm of affordable; we've spent much more on much less.
As I said, it would take the perfect conditions and a champion and I by no means expect it to happen, but I wouldn't discount the possibility. I mean, ten years ago, who would have believed Ottawa would be building a subway through the downtown core?