I think this feature request can be closed, as there already is a feature request about “additional item types”:
If this request here, alternatively, should be interpreted as asking for “(customizable) templates”, then there also is an existing feature request for that:
And right now it is already possible to create one’s own vault item templates: create a vault item, add custom fields as you need them, give that vault item a name you can easily find again (e.g. “Template - API keys”) – and then clone that vault item if you want to create a new item with that “template”.
For all those reasons, I’m going to close this feature request (with a delay for further clarification, if necessary).