In addition to the 18 Tera Jewels for each Type, a select few Pokémon should have a special Tera Jewel unique to them that changes them into a new dual-Type and slightly changes their appearance. Tera Blast turns into a new move that utilises one of their new STABs and has its own unique properties.

This way Terastal isn't left feeling underwhelming compares to Megas, Z-Moves and G-Max which all had special forms for some Pokémon.

(Pic is just a quick sketch as an example).