I love to be able to trigger a Journey via Webhook, but I would love to be able to provide custom parameters to the beginning of the journey.

Lets say I have a Journey for the case a customer purchases a product. This journey includes some introduction emails, an onboarding guide, a set of tipps and tricks and request for product review email.

For very standardized or similar products, I would just like to exchange some bits and pieces of the emails i send out.

A different link to the product page on facebook for the review, a different link to the tips and tricks section for that product etc. 

I could achieve all that with just one journey, independent of the amount of products, if I'd be able to provide some custom data to the journey.

Sure, I could create hundreds of custom fields and a journey for each product, but it is very hard to maintain and with lots of products its a mess. Another possibility would be to use Zoho Flow/Deluge to send out my emails. But actually I'd love to have all my automated emails in one place.

Last but not least, I'd love to have simple data manipulation options within my journey. Like adding and subtracting month, day or hours to/from provided datefield, doing some calculation like ${varname} + 1  or to be able to concat a string.

Edit: Addition: Further you could also support such an option for all the extended email campaigns. For example to be able to work with to content provided by my customer within a Zoho Forms Form. Or using some parameter provided by Eventbrite like TicketId.