Since there's no threads going over these two games and their relations to the Klonoa series whatsoever, I've decided that the time has come to make one.
Radia Senki, released in 1991, is a NES game directed by Hideo Yoshizawa, who also directed Door to Phantomile and supervised Lunatea's Veil. The opening bit features two characters named Gadiss and Princess Lefis in an intense plane chase. Right before the game really begins, the princess ends up crashing her plane after Gadiss bumps it with his.
Several years later, Yoshizawa would reuse these names (respelled as Ghadius and Lephise) for Door to Phantomile. He would also reference this scenario for when Lephise crashes her plane into Bell Tower. Along with this, Radia Senki introduces its protagonists with the player waking up in the woods with their future partner finding them. This could be where DtP's opening sequence originates from.
The other game I mentioned, Wagyan Paradise, was a SNES game developed by a Namco team in 1994. The game has many design choices and gameplay mechanics that most of the Klonoa series shares, even to the point where the design for the Spiker was carried over from the former game to the ladder:
There might be more stuff that I haven't noticed as I haven't finished either of these games yet, but I'll update this thread if there is more info