The age of sexual consent in NYS is 17. The age of sexual consent in Massachusetts is 16 UNLESS the minor is also a virgin. Then if you induce her to have sex with you before she is 18, you could get in legal difficulty in Massachusetts.
The above involves in-person contact. As silly as it sounds, you may be able to sleep with her in NY and perhaps in Massachusetts before she is 18. However, the Federal age of consent is 18. And the Federal government controls the telephone wires and the internet. So sexual chat/phone sex/exchanging photos online is all illegal until she's 18
So to get to your question?
1) is illegal to have a platonic conversations?
No, but it is suspicious, so you'd need to be very careful. Many parents would be concerned about the age gap.
2) is it illegal to plan for a sexual relationship after she turns 18?
It's illegal to have a sexually explicit telephone, text, or internet conversation with a minor. It's illegal also to exchange explicit photos.
Be very careful here. There's a well-known blackmail scam where you start chatting with someone who claims to be 18 and she starts sending you explicit photos or videos to induce you to reciprocate. Then she lets you know she's really underaged, and you get a call from a so-called angry parent, threatening prosecution unless you pay him money.
If you're not interested in establishing a sexual relationship online with a minor -- and you shouldn't be because you can be arrested and convicted -- and she won't verify her age, the prudent thing to do is to move on.