Silver,
I'm afraid one of our biggest flaws in the UK is that we just don't do joined up thinking very well - but I'm afraid the US doesn't either, DEN is the largest airport site in the western world, but despite the huge dislocation moving out to that site caused, where is the TGV there? You'll get a tram in 2015.
So I can only comment in two ways - what is planned, or is being considered, and how I would like to do it, given the combined powers of the Mayor of London, the Treasury and the DfT.
Why hasn't Euston Square been linked before? TfL beancounters will have said 'costs too much, people who want to go there will get there anyway, so there's no revenue gain'.
However, with HS2 plan, a Euston Square link will cost perhaps £5m, peanuts to add on to an overall budget for the station which I expect will be around the £1bn mark. As for heavy bags, I'm more concerned about the constant stop-start, uneven pavements on the link in the other direction - that needs travelators too.
Given my magic want, I'd get rid of the British Library and put Euston 2.0 there. That would also grant me permanent favour with King Charles forever - but it won't happen.