You can't do that without lopping off the bit of North Yorkshire between the two areas. And, as I said, York kept blowing hot and cold over the matter. Better to leave them where they are, politically; I doubt fares from York to Leeds would come down to a per-mile ration similar to those within West Yorkshire.
I don't think there'd be any problem including Church Fenton, Tadcaster etc into the loop. In Fact I'd be inclined to include Selby as well if it chose.
The fact that politicians have been blowing hot and cold shows that its an active idea - perhaps its an idea whose time has finally come.
That said, it would be fairly pointless without sorting out the ridiculous fares.
Trouble is, York isn't anywhere near the urban area that would normally make sense to be considered the 'metro' area. There's about 15 miles of pure countryside between York and the edge of Leeds.
Neither are Walsden and Marsden. It's less about continuous urban area and more about economic hinterland. And York and West Yorkshire cooperate politically already as part of the Leeds City Region.
Personally, I'd argue that there should be a combined travel authority, that brings all the Yorkshire's together rather than expanding Metro with fares across the whole area.
There's so many overlap areas, Barnsley to Leeds or Sheffield, Hull to anywhere etc that it doesn't make sense it all being split up. Otherwise it's just going to be a Metro goes to York, it should go to Harrogate, Metro goes to Harrogate it should go to Skipton. Cheap fares at York, why aren't we part of it in Selby. If Selby are in why aren't we getting this in Malton and so on.
Actually yes, a Network Yorkshire would be a great idea (I might actually get a decent service to Whitby).
It would need a Government that actually believed in devolution, rather than micro-managing from the centre.