It’s unclear what you mean with “a tabbed GUI”, we already have tabs in multiple places. Although I assume you refer to editor tabs, where each area (sub-window) could contain multiple editors stacked as tabs. Possibly in combination with a more conventional docking system.
Implementing such a thing shouldn’t be a huge challenge for someone well versed in the UI code. But it’s a question of priorities too.
A “new GUI code” was never officially planned. bWidgets is a personal long term project of mine, and I hope to find the time to push it further soon. But it’s not a requirement for such features, although it would/should make many related things easier.