https://idioms.thefreedictionary.com/be+on+to+(something)

In this context, it's ONTO something. I think your idea has possibilities/could yield results/has promise/may be a clue to something we've been confused about. For example, let's say that I had the notion that Fëdor is actually Vadim. You may or may not believe me. Then in one of his stupid comments, he writes "Уважаемый американец (кубинского происхождения)" and you say "Hmmm, you may actually be onto something..." meaning "you're idea about Fëdor being Vadim may not be so crazy after all."

Not so crazy... At least, my first thought about him was the same...🙄

...however, I think he just read your profile.

Maybe, but... it’s a good example for ONTO! 👍🏼😝

Anyway!