One issue is single v. multi seat constituencies.
Single is simple, keeps MP link, and is what we have now (and hence default); but it produces samey MPs, and is vulnerable to gerrymandering.
Multi can yield more appropriate representation; but it is novel and complex, and too easily leads to incoherent and/or unmanageably wide constituencies.
I personally would go for multi-seat where towns or counties are now artificially divided, but stick to single-seat elsewhere.