LOL! Personally I'm iffy about this. IGA's Japanese and there were no Japanese in Romania during the 1600s. I just don't think it would fit well with CVII but that's me.
It's based off of SOMEONE's signature "Gabriel and Laura go into a cave and find... IGA"
If changes along the way, I do wanna keep it as a secret gag, most likely accessible only after you beat the game.
Most likely it'll unlock a Playable IGA, probably some 4th wall material, as a way to tribute IGA's contribution to Castlevania and have a few laughs
uzo's suggestion seems to fit better IMO. The Crystal Knights are already in CVII so giving them more of an important role in the game wouldn't hurt, but enhance them and the story a bit. And like he said the rival hunter could be a member of their order. In fact he may have been ordered to confiscate the whip by order of his Grand Master due to Simon's failure, thinking that they themselves are more worthy to wield vampirekiller. But of course the order would not realize that only a Belmont can wield it, thus nothing ever becomes of their plan.
This could go down like this.
Simon leaves behind the VK, travels to Transylvania
Rival Hunter is sent to retrieve VK around that time (possibly tipped off by Carmilla)
Crystal Knights act like a small Brotherhood of light, but formed through desperation