[DTV]

I also suspect that if the “Newsnight” element only went out in England, people would ask awkward questions about that and eventually it would be dropped as a result. It wouldn’t exactly help with promotion or viewing figures either to limit the reach. Viewers elsewhere may be annoyed at not getting to see it; viewers in England may ask why everyone else was getting “more” local news. So it would probably annoy everybody!

My alternative view of an imaginary News at Ten-Newsnight hybrid would be less 'segregated' - with around three stories being given an 'in-depth' semi-Newsnight-esque treatment (extra analysis, maybe one interview per programme), with some standard 2' intro+VT stories being included between them. There might be some concern of a slowed programme, but given how long the lead story can get on the Ten these days, it's not especially radical.

I do agree, though, with Keith's idea for England opting back in, while the nations stay opted out - potentially with their own 'harder', more politics focussed programme (potentially allowing you to 'rationalise' existing standalone current affairs programmes in the nations). Of course, they did experiment with the three nations having their own Newsnight opts in the early 2000s, with only Scotland having any longevity, but they suffered from being duplicative of existing news bulletins and politics programmes (which would not be such an issue in this scenario). 

Separation would also allow the residual English programme to cover England-wide but England-only issues (education, health, etc.), as well as focus on particular regional stories (to make up for decreased function on News channel) and potentially airing the odd regional investigative report (as a sop to axed Inside Out/We Are England).