Weren't the plans for a fourth Olympic class or another large [Titanic replacement or larger than Titanic] ship after [or taken more seriously] after Titanic's loss?
So far as.ive seen there was never much talk of such a thing before the war. Brittanic was too far along to do another dual build... I'm a bit surprised there's no real records of at least considering going straight from brittanic to ship 4, but on the one hand the war started, and on the other with the changes planned for Brittanic I don't think white star was interested in another exact clone anyway... How different they would have gone through...
Now, as things happened those talks in the 20s seem to have been the first serious ones for another ship, but bear in mind that RMS Majestic was functionally brand new and perfectly suitable (and was nominally Brittanics replacement), so even after the war the triple ship weekly express service only required one more ship.