Here's a map of the most practical routes for getting between the Twin Cities and Northfield. Any route to Minneapolis would require a lot of work restoring tracks to good condition, including the bridge over the Minnesota River, and a new junction with one of the lines leading into downtown Minneapolis would be needed -- A project very similar to the upgrades proposed for the TC&W reroute for SWLRT could be required, grade-separating more of the line through St. Louis Park. There are Union Pacific tracks to St. Paul that are in fairly good shape (for freight, at least), though even there, I'd prefer to use the branch that currently ends in Eagan rather than heading through Inver Grove Heights. There's a 4-mile gap in that route, though, now partly occupied by the Big River Regional Trail.
The actual Dan Patch line (in red on the map) was not really built with speed in mind. It has a lot of relatively sharp curves down to roughly Lakeville, but farther south it's fairly straight. However, that might not be a huge dealbreaker if there's a continuing effort to legalize European rolling stock -- there are probably some tilting train designs that could handle the route at decent speeds. Some curves can probably be eased without too much trouble, but many would probably require taking buildings to improve.
I can't say what sort of ridership we could expect with any of these routes, but the Dan Patch route in particular comes from downtown Northfield, goes through the center of Lakeville and runs past the shopping district at I-35 and Kenwood Trail, then passes through the business district around Burnsville Center. Once it hits Bloomington, it passes through a business district along Old Shakopee Road and passes through two clusters in Edina. Depending on how it goes north of there, it could hit another two or more employment areas before heading to Target Field. There isn't great residential density along the route, but if you can put stations in locations that are natural attractors for people, that can help a lot. (Like SWLRT, a lot of the business districts are not very walkable, but things can change with transit-oriented development.)
Unfortunately, I think this type of rail project is very unusual in the US, so there isn't much to compare it to, though I'm not sure that's a bad thing -- I don't like purely commuter-oriented services, since they only work for people traveling in one direction and don't really let you pop into town for part of the day to do a a shopping trip, doctor's visit, etc. In my ideal world, I think most of the route (perhaps not all the way to Northfield, but at least down to Lakeville or Farmington) would have hourly service for off-peak periods, ramping up to 15- or 20-minute service during rush hours.
I'm not sure if a route to Northfield will ever really be able to share much track with a route to Rochester or Mankato, but MnDOT does have a line to Albert Lea on its radar, albeit for implementation sometime after 2030. Faribault and Owatonna would be other intermediate stops, most likely, and clearly it would make a lot of sense to continue the line straight south into Iowa to eventually reach Des Moines.