Oh, I dunno. I personally think they should make the new Euston just as much an awful concrete monstrosity as the current one. That way, when we need to remodel it again in 50-60 years time, nobody will mind if we tear the whole place down again.
If you build it all shiny and nice, people will get upset if you later want to rebuild it. The problem is exacerbated if English Heritage decide it's shiny and nice and decide to list it.
So, largely tongue-in-cheek, but it's a fair point, I feel. I reckon remodelling Euston is likely to be much easier than Kings Cross was.
However with the rumours of Crossrail 3 running between Euston and Waterloo there maybe less demand in 50-60 years time for trains terminating at Euston.
As even if IC type trains run to Euston (Crossrail 3 platform) and then to Waterloo (Crossrail 3 platform) and out again (even if everyone from outside London got off before the train headed back out of London) it would provide a much needed boost to capacity as the trains would only need to turn around outside of London.