Terra and Aqua were together in the Castle of Dreams when Terra mentioned to her that Xehanort was after hearts of pure light, and I think that he was in Enchanted Dominion when he learned about it.Here's my thought on the connections,
So far, TAV for the most part have only visited home worlds of the PoH. One of the few worlds they visited that do not have a PoH is Destiny Islands. One scene implies that Terra may be able to sense power of different hearts, specifically he was able to sense the pure light of a heart of a PoH. Perhaps this ability is what draws them to different worlds in which special hearts reside. However in the case of Destiny Islands, they are not drawn by the heart of a PoH but instead by the hearts of two children which are special in the way that they are qualified to wield the Keyblade.
But what I found interesting was that he said "We'll chase the light" or something along the lines of that. Maybe he, or Aqua or Ven, does have some sort of light-sensing ability.
I think that this is probably the case with Riku, but I think that Sora is something that they didn't have these sort of plans for. LS (I know the fight isn't canon, but what he said must have had some significance) was shocked to see the keyblade in the hands of Sora. I would think that Terra might have recognized him had he been a person as important as Ven's apprentice-to-be.And as in the cycle of Master teach Apprentice, some day it will be the duty of Terra, Aqua, and Ven to instruct these two in the way of the Keyblade. Though Sora and Riku are too young to take on as apprentices, having found these two qualified hearts, Ven and Terra make determine which among the two children will be their students.
There's also the Room of Sleep to take into account, as well as Ven's request: "Erase Me".I think by the end of BBS, Terra and Ven's hearts are drained almost entirely of their light and darkness, causing them to lose some of their heart's strength and most all of their memories. I think Ven's heart falls into Sleep as a result of this, and is able to travel through the world freely. He might go to Sora as part of his duty to guide a future wielder even though he is no longer present in a physical form. Terra barely survives but retains human form. He becomes Aprentice Xehanort and then becomes a Heartless. He possesses Riku's vessel and changes it into one of his image with black white red clothing. When Sora loses his heart, Ven's heart is released and it too obtains a vessel and determines its form into one resembling Ven with black white red clothing. Then Sora beats Xehanort's Heartless and Riku reclaims control over his vessel. However Riku's heart was also greatly drained of light and darkness, which is why he almost falls into Sleep. However he embraces his darkness, suppresses Xehanort's Heartless, and finally the light from Ansem's machine made Terra's heart dormant within Riku. Also, Roxas returns to Sora, returning Ven's heart to Sora, and Ven's heart too goes into a dormant state. Thus by the end of KH2, Terra's heart Sleeps in Riku and Ven's heart Sleeps in Sora.
The last part there got a little bit crazy for me, but overall, impressive theory. Specific, but still reasonable (although you did lose me at the end o.o)