Right now, our league's architecture is not such that we can simply re-enable a previous League, it'd still take a chunk of dev effort for us to do but we do feel we'd need to make tweaks to previous leagues anyway.
There's tons of new content, we feel this should be represented in League tasks, Relics. Especially for Trailblazer, for example, it'd be right for us to include Zeah and make sure the new content is tied into the area system appropriately. That said, we've not decided on the next League yet.