Here are some of my tips for designing Homebrew special characters. Points values can be a real pain in the rear to come up with as it seems GW do not a have a clear formula anymore. So I always try to find something comparable to what I am creating and work from there, often adding a point or two initially for balance sake. Sometimes the extra points stay in the final product, but often they do get dropped. Again this is on the cautious side of design, but it is what I have found to work well.
When creating special characters I almost always add 20 points to the Character type before considering any Wargear or Special Rules. These are the base points I use for stat increases to WS/BS are worth 1 point, S/I/A/LD worth 5 points, T/W are worth 10 points each.
This is the points breakdown for the universal special rules that I use. If the character is a Monstrous Creature add +5 points to the points shown below.
Counter Attack 10pts
Eternal Warrior 35pts
Feel no Pain 20pts
Furious Charge 20pts
Hit and Run 20pts
Move through cover 5pts
Night Vision/Acute Senses 5pts
Preferred Enemy 10pts
Tank Hunter 5pts
When creating custom special rules you have to really consider the impact of the rule on each and every phase of the game. I usually start at 10pts, and then add +5 for each phase of the game that it can have an effect.
It’s not perfect, but it works for me and tends to feel balanced. The best advice I can give if you are going to create custom special rules is playtest them vigorously; once you feel like you have playtested them thoroughly, playtest some more. You need to make sure to get input from your opponents, about how the character feels, in balance, power and points value.
Feedback is an extremely important part of any rules design and special characters are really the hardest thing to balance. Especially in the eyes of your opponent.
I hope this was useful, for you budding homebrewers out there. If you have any questions feel free to ask.