You're right, there is a gap to the northwest of the town. The proposed Northwest Relief Road is scheduled to link across that gap from the A5/A458 junction at Bicton Heath, to the A528/A5124 junction just over 2 miles north of the town centre, and at present the plan is to start construction in 2025, with opening in late 2026. Optimistic, but that's what Shropshire Council presently say on their website. Any new railway linking between the Wrexham and Crewe lines would be well clear of the flood plain.
The route shown by
@zwk500 above ties in well in terms of attracting possible developer funding, which I mentioned in post #50 above, perhaps this might even have slightly more merit than just being a crayonista exercise! I should say that I'm not advocating development here and many would oppose it, I'm simply airing what might be an inevitability that eventually seems to happen near to most new roads.
Of course this would do nothing towards the original objective when this thread started, of speeding up the Manchester to Cardiff trains because it would add a couple of miles to the route, however a new route wouldn't necessarily require the existing route to be closed. The new route would have potential to serve a new Park and Rail station and it would usefully allow Platform 3 to be used for northbound trains towards Crewe, which could be for those trains which are less time-sensitive, ie boht slower passenger trains and freight. Meanwhile, trains where time is more critical could continue to use the existing route, but there would be a very useful net benefit in an overall reduction in the number of trains using Harlescott Crossing.