Hello there, I’m usually handling our GSoC participation, here’s some tips on projects:
Projects should generally contribute “to” the project, such as core things like libraries, the compiler, the ecosystem as a whole, or tools like vscode swift plugin or swiftpm etc etc. Things like forums and apps are generally not good project ideas, it is best to pick an existing problem in the Swift language, libraries, or ecosystem, and pick it such that it is solvable in the few months of GSoC.
If you contributed to sourcekit-lsp, there might be some project ideas in there for example. But of course there’s many ideas you could come up with – the crucial thing is to have a concrete, valuable, deliverable at the end of a project, and making sure there’s a mentor to guide you through that.