An important aspect of this is how your wife sees you. She doesn't see you as (sexually) aggressive because you never have been. You've been (hopefully) a respectful, caring and open husband with her. You're her partner and lover. Her Dom is someone to control, hurt and fuck her.
You can't suddenly shift from one to the other and expect her to react as she would if you had been that way all the time. People don't work like that, and she's probably very apprehensive because of that.
You're also discovering yourself as a Dom. While elaborate and well planned out scenes are fun, I would suggest doing something looser, to let yourself run a bit with your imagination. You're feeling yourself out, and your wife too. Don't be surprised if you find her reactions, or what she is willing to do, a little surprising. The same for yourself.
There's also another important thing to remember, when you're in charge, you're in charge. If you're Domming her, you don't ask. Limits have been established. Her pleasure only matters when you deem it to matter. You are the centre of her world when you're in control, and punishment is for when she does not respect that. Act without limits, act safely, but stay in control. Be firm.
My advice would be this (I have no idea how much you've done already):
Think about your own desires. What do you want from this? Do you want this play to just be confined to the bedroom? Around the house? How far do you want to go in terms of being in control of your wife?
Talk about what you've thought of with your wife. See what she wants. She has to be willing to try and work at this, too, but it seems like she is.
Discuss kinks and limits. You know you're in to humiliator. What's your wife in to, besides being spanked? There are plenty of quizzes and websites (Mojo Upgrade) that can help you if you're a little apprehensive about sharing a particular kink.
Practice and read up on any acts that you're not too sure about. There's a lot of technique that can be learnt very quickly. (Kink University)
Go slow. You want to have a little part of you that sees your wife as a sex object.
Always be safe, and always talk things through before you do them.