One of the best things you can do before building a new product is to start with the problem, not the product.
If you plan to build another form builder, what problem will you solve that the others do not? You should know the answer to this question before writing a single code line.
We did exactly this at Joyfill (http://joyfill.io/developers an embeddable form builder SDK for developers) — a problem was brought to us hundreds of times before we even considered building another form builder.