Interesting idea. Could you color code that differently, so the different routes are more obvious?
Ask and ye shall receive.
I-391 would remain intact as is (not highlighted)
New I-91 is Blue (former RT-5)
New I-291 is Green (former I-91 mainline)
New I-390 is Orange (truncated former I-291)
RT-5 is Yellow, and functions as a frontage road through the commercial area in West Springfield.
Rt-20 is also Yellow, and takes the place of I-291 from Liberty St west as a four lane boulevard.
I also added/highlighted some connecting streets in the former I-291 ROW in violet.
I think the downgrading through downtown Springfield would help as a 4 vs a 6-8 lane mainline is far easier to bury or trench in small phases, and being more narrow is also far more humanizing to have to walk over/under.
The new I-91 would have only three exits vs the current 13, making it a superior through-route. My rerouting also only requires eminent domain at the ends, and only one of those is a residential area. I imagine the best time to do this would be as the current viaduct begins to reach its end of life. Upgrade Rt 5 first, then begin tearing down the old 91.
Edit: Regarding 6 Flags and the train. I wonder if six flags would be willing to buy a ferry to get people across the river. If you built a station and ferry slip somewhere
here both the MBTA and ConnDOT could run specials to it, similar to the ski trains. The biggest issue then would be that the ticket area is on the other side of the park. Perhaps since it is six flags boat they could have ticket booths right on the ferry, and process guests en route.