What are the guidelines for creating a canning recipe?

Nearly any recipe can be canned, the only caveats are that lower-acidity foods and recipes require a specialized pressure canner to kill bacteria, and there is also a short list of ingredients which are never a good idea to can, but many people wouldn't think twice about adding/subbing into a recipe - fats, dairy and eggs shouldn't be in a canning recipe because they'll still go rancid, grains and flours are good insulators and will actually protect bacteria from the heat, and thickeners that people will often throw into a recipe like cornstarch or tapioca will chemically break down from the canning heat. You'll find plenty of cases of people canning grainy/floury things anyway with no ill effect, but that simply means there was no bacteria to kill in the first place. Better safe than sorry.

Most people don't even know what pH is, let alone how to test it or how a recipe alteration would change it. They also aren't going to know much about bacterial growth conditions or heat transfer. It's a heck of a lot easier for a recipe site or book to just say "no substitutions" than to try to teach people how to do it safely, especially when there's plenty of idiots who wouldn't pay attention and sue when they get bacterial poisoning.

So there's really only two guidelines you really have to worry about for your recipes:

1. Avoid the ingredients that shouldn't be canned, or at the very least, understand that you're taking a risk.
2. Foods with a pH level of 4.6 and below can be canned in boiling water. A pH of above 4.6 requires pressure canning. This pH chart gives some examples of commonly canned items.

Sources:

• The acidity rule is a fundamental safety rule that any decent guide will have. Ball's guide mentions it first thing in "Getting Started". Most guides will simply refer to high-acid and low-acid foods. This safety guide from the National Center for Home Food Preservation details the pH reading of 4.6 as the dividing line, which is also visible in the pH chart linked in the answer.
• The dangers of canning cakes and breads have been published by many research departments, including those at Penn State and University of Georgia
• Thickeners such as cornstarch chemically breaking down due to the heat involved in canning is detailed in the Cook's Thesaurus. This breakdown is the entire reason for a product called Clearjel, which is also mentioned on the same page as a substitute for cornstarch
• Clemson University states that no safe methods for home canning dairy products exist.