[Mateo]

Sounds like it would be a good thing to look into, possibly re-work their teams as well. Also,

Spoiler:

There are 4 legendaries available in South Isle if you come back for the Legend Festival. To activate the festival, you need to find Flannery in Fuchsia City and she will tell you about it.

There is also Mewtwo in Cerulean Cave, and there is a Celebi Event in Ilex Forest once you take the GS Ball to Kurt.