-Rapid Spin on Ferroseed and Ferrothorn
Come on Game Freak. The metal ball is freakin spinning in your sprite! You just proved that it is capable of rapid spinning. Even Bulbapedia commented that it's strange that Ferroseed cannot use Rapid Spin. This alone should give it Rapid Spin.
Well, to be honest, Ferrothorn is already a great utility Pokemon without Rapid Spin. Rather adding Rapid Spin would make Ferrothorn the
best utility Pokemon, as well as making Forretress cry.
Nevertheless, it'll mean that Ferrothorn can provided team support even when Taunted and against Magic Mirror Pokemon. It doesn't automatically make Ferrothorn shut down by Xatu. Also, all the commonly used spinblockers would have to be scout Ferrothorn's set before switching into Ferrothorn; Jellicent does not like Power Whip or being seeded, and Gengar does not like Gyro Ball. It's also been said countless times that Rapid Spinning is hard in this metagame since the only good Rapid Spinner added in 5th Gen got banished to Ubers; Ferrothorn would help mitigate this issue.