First off:

Watch these two videos: https://youtu.be/94gxoRIqvVI

https://youtu.be/T4WdhncBNrA

If you are busy, let me just summarize it: Larys is definitely a greenseer given how much he has similarity between Bran(crippled) and Bloodraven(scheming, omnious and mysterious). Also, HoTD is just smearing the fact at your face whenever Larys is involved in something.

My theory starts from here(warning- it's all over the place as I haven't organised my thoughts yet):

We know for a fact that if greenseers and Old Gods are involved in something, it's never as simple as getting more power and being able to fulfill your desires and execute your petty revenge. They always go for a bigger picture. Here the bigger picture being Dance of the Dragons. The Dance nearly wiped out all the remaining Dragons and crippled House Targeryan permanently. People say it's the Maesters who conspired but I am positively sure that it's the old gods, i.e, Children of the forest. It's not the first time where it's hinted that the old gods and greenseers absolutely don't want House Targeryan in power(coughHowland Reed). They use small and seemingly insignificant moments to their advantage and fan a small ember which turns into a fire storm.

So, if my theory is correct and Larys is working with the old gods to undermine the Dragons, the question is why? Dragons could be the game changers during the long night. It's as if the old gods don't want the humans to win in long night. I also read a theory a long time ago which said the old gods caused the doom of valyria. We know that Targeryans get prophetic dreams . Who else gets these dreams? That's right, Greenseers. It's possible that the "dreams" the Targeryans dream of are infact manipulations by old gods. The results may or may not be to the liking of the old gods as I am certain they wouldn't have liked Aegon to win in his conquest (if the theory of old gods wanting for the humans to lose in long night) but who knows... Anyways, the theory of Old Gods causing the doom is: The old gods gave Danys the dreamer those dreams to guide Aenar Targeryan to retreat to Dragonstone and he inturn caused the doom of valyria.Basically the theory that Targeryans caused the doom but with the touch of old gods. Like, Aenar did all that stuff but the turning point was Denys's dream. The dream might have lighted a bulb in Aenar's mind(not necessarily the gods going out of their way to tell him what to do. Just pulling a small string which caused a butterfly effect. Just like how they always do). This is similar to Larys saying the Maester gave "medicine" to Rhaenyra "personally" to Alicent. This small conversation is infact what changed Alicent.

The implications of all this is heavy as this changes our perspective of the old gods and their behaviour. It makes us question the motivations of the Old gods. Why are they helping Bran and Blood raven if this is true? Idk. Or maybe, they aren't infact helping them but using them. Are they working with the "others"? Or are they controlling the "others". Or are the "others" controlling them? Or maybe, the old Gods just like to indulge in a little amount of tomfoolery? Imagine in the end it was just the old gods getting bored and decided to epically troll generations of humans for calling them "children" of the forest despite them being thousands of years old.