You can add borders, watermarks, seals and I think the course instructor signature block. Those are images, I think you can create different certificates for different levels, or courses for that matter. Being really creative, you could create a "language" for each of your levels. If I read Certificate module instructions, some of the print is editable in the Adobe file.
See you have also discovered the Certificate forum. I have not touched base with Chardelle in months, but while busy, she has been helpful.
Thanks for this post, it caused me to look at the Tip and Tricks in Certificate that talks about Lesson. I think that applies to an "older" Moodle site and I will have to think about that one. Wonder if I wrote that back in 1.7 or so
Hope this gives you some more ideas. Your idea with Lesson will work but...
Chris