This is a cool project, and I love that you're looking into other dietary restrictions! Gluten and dairy substitutes would have been very useful to me starting out, but after nearly a decade I tend to make omissions rather than substitutions. I've seen others upthread mention that they prefer to find dishes that are naturally vegan rather than use substitutes.

What I (and, possibly, they) could really use would be something that would suggest an alternative recipe or a related dish that works better without substitution-- e.g, a similar recipe that doesn't rely on the meat or cheese for flavor, or which doesn't rely heavily on bread/flour/sugar/etc. as a base ingredient.

I expect it'd be more difficult to reliably identify similar recipes AND restrict the search to avoid some set of ingredients, but it would be a very useful resource for people looking to omit certain foods without resorting to (often expensive or unhealthy) substitutes.