First of all, I'm sorry you have to go through this.
I'm a step-mom, so I've dealt in depth with this issue from that perspective. It's hard to give suggestions without knowing more about your circumstances, which, I would understand you being reluctant to expand on. But, please ignore any of this that doesn't apply. (For example, when you say you have custody ,I'm not sure if you mean mostly or all)
Here are some general things I've learned from dealing with our plan and helping a friend propose his:
1) Try to keep it simple. You'll want to balance out holidays and birthday, but try to picture how your week will look. Don't make switch times disruptive, during dinner or close to bed-time, for example.
2) Try not to have to count on the other person. There are multiple deadlines in our agreement. Ex-wife is consistent about choosing things on the very day of her deadline. Within her rights, and, I'm sure just done out of convienance rather than malice, but its maddening to try to be planning summer vacation, when she can wait until June 1st to declare her days.
3) Will there be religious differences? Our order states that a parent cannot interfere or refuse to allow the other parent to expose the child to their religion. I think that's a nice provision. Either way, if its an issue, it should be addressed.
4) Are you kids old enough to travel yet? Dh was divorced when DSS was a baby. Now, he's older and family trips are fun. However, weeks get so chopped up that its hard to plan time away together. Just consider the balance of not wanting kids away too long vs the disadvanage of not having blocks of time with them.
That's all that comes to mind at this point, but if you want to PM or post with anything more specific, I'd be glad to give you our living with it perspective.