You can always make up a symbol (I've done so on several occasions), but, like user4594 said, whenever you first use it in a work, you need to always precisely define it. If you find yourself needing to use the word association or correlation a lot, and you don't want to use existing notation (e.g. corr(A,B)), then you can insert a symbol and either make a footnote that defines the symbol or include a legend/key at the beginning or end of what you are creating.
Using a symbol for every relationship between arguments often unnecessarily burdens your reader, but sometimes, I find that it helps me remember things better if I can assign symbols to certain words, especially if I end up using the words repeatedly in a single work.
Here is an example of a group of symbols I have created (not designed, but assigned):
"⌽A, ⦵B, ⍉C, ⦵D"
...meaning:
"If A, then B, else if C, then D."
I know that "A⟶B" ("If A, then B") is more commonly used, but in my particular document, a ⟶ meant something else entirely.
If you want to make a novel symbol but don't want to personally design one (font editors can get really complicated), there are some fonts you can download that consist mostly or entirely of symbols (many of which you will probably have never seen before). Some examples, at least in Windows (I don't own a Mac or Linux machine), are Cambria Math, Wingdings (including 2 and 3), Segoe UI Symbol is quite extensive (it even has symbols of playing cards and dominoes among other things), as is Segoe UI Emoji, and Symbol. The font Segoe UI Historic has a lot of random characters that were probably part of some ancient languages, like (click here if you can't see them). There are plenty of options to choose from. If you use an obscure symbol, and you want to send the document to someone else, either send them the font file you used (impractical) or save the document as a *.pdf or image file, because if the other person doesn't have the font installed on their device, they will probably just see boxes.
Usually, though, I only do things like that when I'm making notes for myself (I use plenty of custom shorthands when I take notes). Neuropsychological research has suggested that the human mind has a rough limit, reported as anywhere from 3-7 distinct items, to what can be held in working memory at a time (Cowan, 2010), so it may be counterproductive to use too many symbols, as your ideas will be lost to your readers while they try to keep all the symbols straight. It's probably a good idea to apply the adage: when in doubt, just write it out.