I'm sure someone else was plotting way before Robert's Rebellion. ;)
Ok, you've got me there. The prequel "history" to the Game of Thrones period is dominated by the Blackfyre rebellions. But I don't see a thread connecting Blackfyre survivors to Aerys' happenstance madness to Robert's Rebellion to Varys' efforts at a Targaryen restoration.
The Others have been laying in wait, but I'm not sure "plotting" is their style.