You're looking at the following criteria, apart from manpower and funding.
1. Proximity to greater Belfast or Dublin areas, or Cork at an absolute stretch. With Downpatrick in the former category, we're now just looking at Dublin. So counties Dublin, Meath or Wicklow; maybe Kildare.
2. Good road / parking access and proximity to main roads. Access needs also to allow a low loader to bring rolling stock in. No use if it's down some country boreen.
3. Planning permission for staff and public car parking, maintenance sheds / covered accommodation for stock. As Whitehead and Downpatrick will attest, it's a fool's game to leave rolling stock outdoors in all weathers.
4. Sufficient length of line to give the public something like a journey; half a mile is useless, 10 miles is too long to be economically viable in this country, DCDR's 2.5 miles is about right from the point of view of fuel economy when operating; and ease of maintenance. Double it to 5 miles, you double the coal or diesel used, but you won't get away (in this country, anyway) with doubling the fare.
5. Somewhere to go to. A train ride from a small unremarkable rural halt to a buffer stop in a field is useless, as far as sustainability is concerned. It has to GO somewhere. Downpatrick has Inch Abbey. One scheme considered in the past, Trim to Athboy, had a heritage town near a motorway connecting with another in which there is a fmous castle. That sort of thing.
As enthusiasts, we're all good at the "ideas"; the "what-ifs"; and what WE think would be interesting. But "normal" people - who pay the bills (enthusiasts won't) do not think the way we do. Cold hard reality must always take precedence over "it would be nice to preserve XYZ". As a diehard, hardcore, proper nerd-level enthusiast myself, but who spent 25+ years dealing with day-to-day management practicalities of both the RPSI and DCDR, I have seen chapter and verse, plus the full boxed set, on all of this!