Tthe "emotional maturity" Kondo commented on in Agate's Dengeki reveal could point to a different outcome for their relationship, where Agate asserts his big-brother-bodyguard position but stops playing dumb to Tita's love for him. He's the kind of guy who'd point out how he can't be her romantic end-goal if that makes her more dependent and less able to find her way in adult life. And he's the type to push love and affection aside for work, but there's people like that too in the Bracer business who might catch his eye.
As for 3rd, only Josette actually wants to see a romance between Tita and Agate, while the Russell adults (all of whom aren't willing to consider Tita's positions on many things, including Orbal Gear) silently resign to the idea she's going to drag Agate into romance. The writing tone is more ambiguous in these moments, suggesting more that certain characters have a warped enough view of the world (Josette being a noble girl who expects large age gaps in high society she used to inhabit, and the Russells who all proudly think Tita will become tenacious and uncompromising like themselves) rather than Falcom itself. And honestly, the introduction conversations in 3rd are half comic relief and half for development, so it's a tough call to make; the Moon Door stresses how Agate sees her as his new little sister, nothing more, and Tita even agrees with him.
People need to stop saying this is "light novel" shit that Falcom suddenly decided to embrace. This is Falcom shit. They laid the groundwork for this themselves. This is their trope.
To say they don't take influence from other media, domestic and foreign, is a reduction too, but Falcom's writers have preferences of their own to balance with what fans expect from certain characters already established. I don't have any data on what Japanese fans think of Tita and Agate, but there's going to be some resistance to the status quo for these two which, coupled with negative impressions online about Rean/Elise (in Japan, supposedly), should make them think of a specific solution rather than playing a trope straight.
Playing too many controversial and ultimately unrewarding tropes straight has been a problem for their writing team lately, or maybe Ys VIII mostly avoiding the problem means they know they've gone too far.
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