Monolith Soft is extremely busy.
What once started as a lower key RPG house has grown into a giant with multiple development teams, who don't necessarily have the time to dedicate to flight of fancy projects. Series director Tetsuya Takahashi seems to feel the same way, as he just recently spoke to Vandal about how much the Xenoblade series means to them now, and how it's tough to divert their attention elsewhere.
Speaking frankly, Takahashi stated that he'd "like to do a smaller-scale project if the opportunity arose," but right now the focus of the company is to "[increase] the value of the brand that [they] have created with the Xenoblade Chronicles saga." He further goes on to say that if they "manage to organize ourselves in a way that allows us to do so," it could happen. But for now, all eyes are on Xenoblade, which has hosted three proper main entries, multiple remakes and one big DLC expansion to date.
Takahashi also commented a bit on present matters, like why Melia was chosen to assist Shulk in the new Definitive epilogue. He explains that because she evoked such a fierce amount of empathy from people who played the original she was a perfect emissary; not to mention that she was one of the more complex party members of the bunch when it came to her backstory.
He also says that the epilogue map (Bionus' Shoulder) was partially a "test map" from the Wii edition, and that the decision to restrict the party member was deliberate, so everyone "gets the attention they deserve." Takahashi also reminds us that the Nopon race are the mascots of the Xenoblade universe, so they had to make it in, as they "are part of the essence" of the series.
While Takashi is busy, he still seems to have a ton of passion for the series and is at least doing what he loves.