Well, for a start, looking at the existing actions (create contact, or create task) they need to be able to mimic the existing functionality within the normal client.
For example when creating a contact there is no “Contact-level Invoice Sequence” feature, no default terms, and so on. Merely having these would be good. Extending these to “request direct debit mandate on creation” would be excellent, too.
However one thing that I have had to code my own deployment for is because you allow new invoice as a trigger, but not as an action. So if I have my billing system elsewhere (as I do) create a new invoice, I cannot use Zapier to synchronise it over. This nearly resulted in us finding another system but many months of work with Kate, your product manager, saved us from having to do so.
tl;dr: contact-level invoice sequencing on contact creation screen, please