email on registration showing the plan

Those won't send until their account is active. So if you require confirmation or approval they won't send until confirmed or approved.

I am pretty disapointed,
this feature looks a must-have,
If you feel a form, the email generated should be able to read the data sent...

I will try to make changes in the core,
any tip to start?

Notice that even when we approve the user (not the plan), the other email (you applied for a subscription...) is not sent either

I will try to make changes in the core,
any tip to start?

We do not support core edits and can not help you make them. If you need CBSubs information outside of CBSubs, like in your Pending or Welcome registration emails, you can try querying for that information using CB Query Field. Example can be found below.

www.joomlapolis.com/forum/277-cb-paid-subscriptions-support/236506-sell-tickets-and-provide-unique-identifier?start=6#293607

Notice that even when we approve the user (not the plan), the other email (you applied for a subscription...) is not sent either

They have to be confirmed and approved. The CBSubs pending email only sends if they've a basket pending payment.