Hi,
I see too different way to implement this feature.
The fist one is to use HikaSerial and add a new feature to assign a serial to a user just after his creation. It could be an option for the packs, so you could assign several serials in the same time.
The main problem will be the email because HikaShop does not send every time an email to the new user (only in a speciac case, when the user has to be activated) so we will have to add a specific email in HikaSerial and add an email manager into HikaSerial.
The second solution is to use a custom plugin for HikaShop.
Using the trigger on the user creation, the plugin should generate a coupon and send an email to the new user.
Because it is a custom plugin, the development will be a little bit easier ; a lot of elements could be "hard coded" in the plugin itself. A little bit less flexible but you could modify the plugin to change your content.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.