As for the Trillium Line and possible future capacity concerns going downtown and second tunnel, I have an idea.
In 30-40 years, if the Confederation Line starts to get a little clogged, we can extend Trillium into the southern part of the CBD, like this:
I put it on Lisgar Street, terminating at uOttawa. I think that's a good spot.. north enough to serve the existing CBD (so that Trillium passengers will remain on the new line instead of transferring to the original line Bayview), and far enough south to not be redundant and allow for future growth in the Centretown area.
I imagine that the tunnel would start between Gladstone and Bayview, temporarily using the space the MUP goes through to dig through a portal. By the time it gets to Bayview, it will be underground. The black line is a 180m underground concourse that can connect the two lines at Bayview.
The tunnel would terminate at a new underground level of uOttawa station. Going forward, the line could be extended eastward into Sandy Hill & Vanier. Having it interline to Hurdman could be cool but probably unworkable as the line would be deep underground at uOttawa and there's no room to bring it back up to surface to meet up with the Confederation Line.
I don't think this will be needed for another 30-40 years or so, but IMO, they should put something like this on the books for now so future development doesn't impair our ability to build it eventually.