[Multiple Tribes]

This could work as when you move islands and leave some behind, they become two separate tribes that you can both play as.

[Including how many nichelings you want to start with]

I think this would be a great little option for a sandbox mode, although being able to start with up to 15 sounds like a lot, especially considering how it could potentially give you quite a lot of gene variety to choose between right from the start

[Export Nichelings and import Nichelings from other players]

It could be cool if imported nichelings turned up as wandering nichelings at a random time soon after you import them. I also think a kind of 'plaza' where you can view imported nichelings or save your favourite nichelings to export might be a good way to implement an exchange thing.

I'm generally not a fan of content in games requiring you to trade online to unlock, but if it was really easy and accessible I could learn to like player to player exclusive genes.