I wanted to know what is the correct collecation for (for example) significant other who has not come to someone's life yet( an unknown person or a probable person who gonna come in the future ). Is it to say immediate significant other or prospective significant other? Or it might be something else. Thanks
You want the word future.
I don't know what my future wife will look like, but she's got to be making good money.
Hold on a moment guys, but I think my future boyfriend just walked in.
This works whether it's a defined person or not. You can call your fiancée "my future wife", you can state that someone you've just met is your future wife, and you can talk about a completely abstract person as your future wife - the as-yet known person that you will eventually marry.
For a specific person that you're considering dating or trying to date, or who's trying to date you, you might call them a prospective boyfriend or girlfriend. You would usually use that about a specific person where there's some active question about them becoming your (or someone else's) boyfriend/girlfriend/whatever. Literally, it indicates less certainty than future, but people will use future when there's no certainty whatsoever. It's sort of joking, except not (I'm finding it hard to explain). If you say "future", you are literally asserting that you will definitely date/marry/whatever that person, which makes sense when you're engaged to them or talking about an unknown, abstract person. If you use it about a specific person that you aren't yet involved with, it's a sign of (possibly joking) confidence).
A prospective partner is a person that you might end up with, someone you're considering dating or trying to date. You might say that you're looking to meet prospective partners. A future partner (or your future partner) is the person that you will end up in a relationship with, even if you don't know who they are.