The evidence that names with a ZO suffix might be brandable came mainly from this article posted in 2013.
tldinvestors.com/2013/07/brand-bucket-sales-inside-the-numbers.html
I realize that article is old but I thought some of the info might still hold. I basically generated a list of ~17000 5 letter domain names that are not registered and that end with the following suffixes:
['oo', 'sy', 'us', 'ia', 'er', 'ad', 'es', 'cy', 'ee', 'ed', 'en', 'ar', 'fy', 'or', 'ty']
Now I have been going through them on my spare time trying to pick "brandable" names. A lot of guess work though.
I've also cross referenced about 8000 with googles keyword tool and there are a few with ~1000 avg monthly searches. One even has 2400 monthly searches. Would that effect the value of these domains?