Yeah, sure! Here's a few basic things:
- Make their personality at least a tiny bit balanced. Tigerstar had a love life and loved his kits, so did Mapleshade. All of them had real, non-evil emotions. Speaking of emotions, don't make them the cliche, 'ruthless, evil, cunning, malicious, bloodlusting' type personality with no redeeming qualities. If you want that personality set, balance it out.
- Don't make them have a half-baked motive. All villains have motives, and it's poor writing to have something un-meaningful. A great tip is to imagine yourself as the villain. If something to you seems too ludicrous, change it.
- Make the villain have an effect on the storyline. Too often I see villains not actually affect the plot and if their presence in the book was gone, nothing would really change.
- No one is born evil. I'm not saying there has to be a catalyst for the villains behaviour, but there should always be a culmination of things that causes them to act like they do. However, only one thing should not be the sole reason for their actions. One event does not change a good, normal person into someome 'evil.' An example of this would be Mapleshade. She had multiple events drive her to becoming a killer. Getting kicked out of her clan, her kits dying, her mate rejecting her and her mate picking a new mate, among other things.
Also, tabbies are cute and you can definitely make them a villain. However, a brown tabby names Lionfang is definitely a copy.