Replacing Target Field Station with an underground station seems really expensive, unnecessary, and unadvisable.
I do hope that in the next decade, especially with the inevitable inconveniences and challenges of two through-running LRT lines on a downtown surface street, that discussions heat up about a downtown transit tunnel. But I think the more likely result is a tunnel that goes underground between 2nd and 1st Ave, stops twice (at a new station between Nicollet and Hennepin, and beneath the existing Government Plaza station), and emerges at the current US Bank Stadium Station.
That shallow tunnel would be less than a mile long, and could be built with cut and cover and a temporary disruption of through-running service downtown.
In contrast, a tunnel through North Minneapolis would have to be bored, and I think would have to emerge before the I-94 trench and continue on 7th St at grade before climbing to Target Field Station. That would be pretty expensive, although I think you could save a little by digging out a station on the MPS parcel at Plymouth/Emerson-Fremont instead of mining it.
I think this makes a lot of sense, and is probably the most realistic scenario for how we'd eventually deal with the capacity limitations Downtown (if we ever do so). For that reason I'd like to see elevated rail play a bigger role in the discussion of Bottineau alignments (not including the core of North), as it's much cheaper and a much more natural connection to the existing Target Field Station, and all the surface routes I can imagine west of TF involve a lot of awkward intersections. (It's a shame that MnDOT is apparently committing to maintenance of the stupid North Loop Viaduct, which could have been adapted into an excellent ROW between Target Field and North.)
A few nitpicky questions, if you don't mind:
1. Wouldn't it make more sense to include US Bank Stadium Station in the underground part, and then only emerge as close as possible to the Blue/Green Line split? There are still a few grade crossings after that stop, and it seems like, to really get the capacity benefits, we'd want to make sure the entire shared segment is grade-separated.
2. I imagine the brief at-grade segment near Target Field would then become the limiting factor for the system. Have you thought at all about how to mitigate the impact of trains having to cross the 2nd Ave/5th St intersection? Or for that matter, the general mess that is gameday foot traffic around the stadium?
3. What makes Downtown an appropriate candidate for cut-and-cover but not North? Just residential impacts during construction, or something more technical?