Seven was a great concept
for a character, but she was sabotaged with the idiotic catsuit. That undermined any credibility she might have had as a real character and just made her an object and a smutty joke.
The right way to do a character like that is to envision her being a male character. The core of the character shouldn't be sexual, since that's not the core of the Borg. The core of the Borg is that they represent a threat to individual human identity, and that is certainly more than enough for some great stories. So if you want to cast a woman in the role, fine, if she's really the best actor of all the ones you audition. But gender shouldn't be on the casting sheet.
Kes just didn't have the dramatic oomph that any Borg, male or female, would have brought to the show. In retrospect, a male Borg character would have been better, if only because there would have been less temptation to turn the character into a joke. But if the producers had shown some good taste and restraint, there's no reason Jeri Ryan couldn't have been perfectly good as well.
was a decent enough idea for a show, and hey, it had a actor that turned out to be an Emmy winner! So someone in the casting dept was on the ball anyway.