I don’t think that would be the case — for example enforcing implicit none would only break codes that use implicit typing and do not use implicit double precision ..., which is just a very minority of codes, and in those you just put in a simple option (one line) into fpm.toml to turn the default implicit none feature off.
I agree. We don’t need to wait for GSoC. @epagone, why don’t you go ahead and implement the implicit none flag? Send a PR to fpm and tag me in it. Let’s take it from there. I’ll help you if you get stuck. I don’t like waiting for others to do the work, as then it never gets done. 
We are meeting tomorrow with @lkedward, @kubajj, see here: Handling compiler arguments in fpm project - Blog by Jakub Jelinek - #20 by lkedward, why don’t you join us @epagone? We’ll brainstorm how to implement it, and then you can implement it, and @kubajj can then integrate it into the bigger picture. But from the user point of view, we don’t care how it is implemented, as long as it is done one way or another. That way we get things moving, which is the most important part, to make steady progress.