I built a plugin to connect WordPress with MailChimp (https://mc4wp.com) while hospitalised in Vietnam a few years ago and it has been selling well enough to support me in my travels & now back home in The Netherlands from day 1 (right after I started offering a paid version, that is).
I'm also thinking about developing my first product and wanted to start with wordpress, I think a Saas would be too ambitious, how did you start promoting it?
Starting out with WordPress has made a lot of things, especially when it comes to marketing, very easy for me. There's a free version that is used on over 700.000 sites right now and it got there mostly organically.
That said, if your plugin does anything visual, be ready for a lot of beginner questions. Some of your non-paying users will be the hardest to please, especially in WordPress land where you as a (real) developer will be severely underappreciated.
As stated, make sure you create a free version first and put it in the WordPress plugin directory. Most of your traffic will come from there. Make frequent updates, include good keywords, and if there's a need for it, people will find it. It's also a great way to start testing for premium features.
Talking about it in as many places as possible - AWP on Facebook, Post Status, etc. But past that, a little optimization and if people are looking for it in the repo, they'll find it.
