The Sailfish Weather app is open source now, as highlighted in this announcement. So you can contribute things directly back into the source code.
It should be fairly straightforward dev work to create an adapter and pattern each weather source so they are interchangeable from the rest of the codebase’s perspective. (free ones, paid ones (insert your own API key), doesn’t matter but i’d start with the most popular sources). Then implementing any new weather source is simply matching the same interface.