It's been a while since I posted a fantasy transit system, so here it goes. I'm addressing urban needs/problems here, so I'm not really dealing with beyond the Greenbelt.
First would be an inter-provincial link (a blue line "Champlain" or the "Constitution" if Quebec signs it

) that aims to remove most of the STO buses from downtown Ottawa. It links the main federal employment centres in the core (downtown Ottawa, Portage, Chaudiere and Tunneys) as well as UQO to the Confederation Line and Rapibus system. It would start at a station beneath George street parallel to the Rideau Station being built with an underground link beneath William. It would travel on a shallow cut-and-cover under Dalhousie, then emerge onto a slightly widened M-C Bridge, go through downtown Hull via the Jacques Cartier park, the museum, office complexes and UQO, then descend into a tunnel to Tunneys Pasture. While this might seem redundant with the existing PoW bridge, it eliminates a potentially messy transfer to and from major employment centres, and prevents overtaxing the Trillium line. The river is really shallow here and has a good rocky base. In the future it could be extended down Parkdale toward Baseline with a stop at the Civic Hospital
The second line (an orange [insert name here] line) which would be an uban-focused metro that weaves through all the universities and colleges, Lansdowne Park, Bank Street, downtown Ottawa, Lowertown and Vanier. This would be a combination of bored and cut-and cover tunnels with some surface sections, from Blair to Bayshore.
Another thought I had was that since the Confederation Line would have split ends, the Airport could actually become a terminus of a southeast spur that goes down the Alta Vista corridor. the routes could be interlined as Kanata—Airport and Orleans—Barrhaven
Fire away at it!
Downtown detail: