How plausible is a substantially lengthened and widened Lion-class battleship?
Short answerThose changes amount to a new hull, so their plausibility depends on a mission that justifies the redesign.
NotesThe proposed Lion changes are too extensive to count as a modest lengthening. The result is a new hull with a new set of design balances. Plausibility therefore depends on why the navy wants the extra length, beam, and draft and what mission those changes serve. Starting with the operational requirement is the only way to judge whether the resulting ship would actually be built.