I know that as of the beginning of this year the football club announced that it was in discussions to build a new purpose built stadium holding 5000-8000 fans. Locations were vague "the north of the town". A stadium holding up to 8k is much more likely to be affordable and maintainable for the club and as long as there is space around it then I could imagine that something to hold 15k could be achievable. 25k was a stretch. The town is home to 93k people. There isn't much of a potential catchment area outside the town that doesn't have football loyalties to Boro, Sunlun or Mags (and much of the town myself included is already spoken for). Barnard Castle perhaps (pop less than 6k).
That said Darlo is growing, lots of building, including that adjacent to the Arena but also along the northern limits of the town and to the west. But the way that the town centre is dying they might want to think of putting the football club back into the heart of the town. As it's Friday night and I'm at a loose end here's a couple of possible locations
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Location 1, is the old Skipper's car sales site now a largely unused car park, probably a bit tight for facilities.
Location 2 is the location of the old Marks and Spenser store and several others at this end of town probably only half the units are occupied and the others never will be again. So do a "Stockton" and knock it all down should fit a nice 10k stadium there. Easy transport links lots of car parking and on all bus routes, easy walk from the station etc.