I think I have found a possible solution/explanation for the puzzle. My answer is the symbol =
All of the symbols in squares symbolize a letter.
By reading the plain diagonals from left to right we have :
First diagonal
i -> i
chinese symbol which derives from "qi" in chinese -> Q
=> iQ
Second diagonal
c -> c
alphaČ (greek letter) -> a
sigma (greek letter) -> s
h underscored -> h
=> cash
Third diagonal
kappa (greek letter), can sound as a 'c' -> c
rho (greek letter), sounds as an 'r' -> r
y-2 -> y
pi (greek letter), sounds as a 'p' -> p
=> cryp
Fourth diagonal
chinese symbol which means "tea" -> t
omega (greek letter) -> o
=> to
And finally the word "currency" can be easily read clockwise in the other squares.
=> currency
Assembling these words gives the sentence : iQCash cryptocurrency
Considering the '?' symbol is missing in the middle of the sentence, the most logical value would be '=' as it also represents a solution of a problem :
iQCash = cryptocurrency
To sum up in a picture :
![](https://ip.bitcointalk.org/?u=https%3A%2F%2Fi.imgur.com%2Fde5Gvg6.png&t=663&c=yHe0IdpGxYWrfQ)
I hope this is correct ^^