You can't patent such idea - you can patent certain way to achieve it (i.e. certain security methods, etc). Anyway, thankfully, there are many Open (licensed following terms of Open Software = can't be patented by some sh|t company) methods, that can be used without fear of patents. MySQL, PHP, just to name few.

Still, honestly, I understand that You're not willing to start such a project. It's quite a huge task, and Your starting idea was to write Open Application for using external possibilities, not creating new ones. On the other hand, many great projects with many people involved, started as small ideas or quasi-unrelated projects...

/Estel