Even though Game Arts developed Project Sylpheed it was more like a spiritual successor to the Silpheed series than a sequel. Since Silpheed Alternative uses the same spelling, perhaps, its a follow up to Silpheed: The Lost Planet.
While Game Arts has not announced Silpheed Alternative, the publisher filed a trademark for the potential title in the USPTO.
Anyone looking forward to another Silpheed game or another Treasure / Game Arts collaboration?