Also, as a side note, there is no apostrophe used when referring to more than one "EX." An apostrophe would indicate posession in that case, such as:
"That EX's ability is totally broken!" So, "I am so glad EX's are coming back" begs the question "EX's WHAT?" Anyway, here endeth the English lesson. Please stop making my brain explode!
Except it doesn't sound right to the reader. EX's sounds it it would be pronounced e-x-is. EXs sounds like it would be pronounced x-is. At least that's how it is to me. It's a made up game mechanic, no need to get technical on the grammar.