Quite a few obvious guesses coming up considering the answers so far (although they're probably not gonna be the ones you're looking for) Here's two guesses I thought of:
AKHTBN iff the first and last letter (or just the last letter) appears more than once in the whole word, orr;
iff there is two or more of the same consonant or vowel in the word.
Otherwise, time to guess more koans:
ATHENS
CORONA
APOEL
NICOSIA
ELECTABLE
VALERIE
YAY