I'm a bit surprised by this discussion, as the Northern line seems pretty convenient to me if you need to get from Waterloo to Euston/Kings Cross. If I was making that journey across London with bags, I can't see many circumstances in which I wouldn't use the tube. Thameslink - as you imply - is great IF you're coming in to London Bridge, not Waterloo.
Speculating a bit, Blackfriars isn't that much of a walk from Waterloo, so maybe that's one way of doing it if you need to go Waterloo-Kings Cross/Euston and you're dead set against using the tube (not sure how entry/exit at Blackfriars works for ticket validity though if it's a national rail ticket with cross-London tube transfer included)
The Northern is indeed fine between Waterloo and Euston - but KX/StP is on a different branch of the Northern Line from Waterloo; in that case (Waterloo-KX) far and away the easiest is the Bakerloo/Victoria route with the cross-platform interchange at Ox Circ.
For Waterloo-Euston, if you don't want to use the tube, there are several direct bus routes (68, 168, 59). The idea of doing that via Blackfriars and Thameslink is
really bizarre - in the time taken by the walk from Waterloo to Blackfriars, plus the walk from St P to Euston, you'd do more than half the way from Waterloo to Euston anyway.
Hmm, closest I can find to a single-hop route is the number 68 (or 168) bus — you can take that from Waterloo Station to Upper Woburn Place (just off Euston Road) and then walk 11 minutes to KX. Total journey time 26 minutes …
You can also walk from Waterloo to Southwark station, then take the 63 bus to King's Cross from there. Total journey time 30 minutes by that route though.
Fastest public transport option is still the tube with a change though (Jubilee -> Green Park -> Victoria) … 19 minutes.
Bike beats everything very easily, at 2.3 miles / 15 minutes, though. Walking a poor option at 51 minutes. Someone must make draggable luggage that can be used as a bike trailer by clamping it to the seat post, I assume. Would be rather handy for using with bike hire / e-bike hire, when in a city without one's own bike (and with some luggage).
I presume these routes/timings are off of some hopeless website system without human intervention to turn them into sense? For Waterloo to Kings Cross via the 63 bus you'd never go via Southwark station [unless you wanted to go 1 stop on the Jubilee Line for the hell of it, with a hike to and from platforms at each end ... and slow down the journey even more]. From Waterloo you'd head along Stamford Street (same as if you were going to Blackfriars) and - from the main entrance to Waterloo - reach the 63 route as quick if not more quickly than you'd get to Southwark station;
and you'd be 2 or 3 stops nearer KX on the 63 by then.
And taking into account ease/speed of interchange en route, not to mention differing accessibility of different tube platforms at Waterloo, then Bakerloo and Victoria (via Ox Circ) beats Jubilee and Victoria (via Green Park) any day.
Getting any of the 3 bus routes from Waterloo to Euston, if you get off at Upper Woburn Place as suggested, then - if there's a 91 coming along behind - use that and it'll get you to KX/StP without the walk.