If you have had success in the SEO industry, then you should be able to add other services to your product mix that complement SEO quite nicely. Here are just a few areas you could expand into:
If you don’t do any of these things currently I would suggest adding in a couple that make the most sense for what you do and learn from resources that are available to you online and in print.
The one area that’s difficult to “productize” is organic SEO. Some people price by keyword, some have a general retainer, and some just do custom quotes only. I don’t see how you could fairly price by keyword given the radical differences in competition, ROI for the client, the actual client, and so on. My suggestion here would be to at least set some kind of floor pricing (Campaigns starting at $999) or whatever.
If you are just doing consulting on organic SEO that’s a bit easier, you can just set an hourly rate. However, given how everything is intertwining these days I think you’ll find that, for client work, sustainability will be found in doing the actual work for the client to some extent.