I noted that only 2 forms of input are supported by custom recipes.
Is it possible for me to have more flexibility, such as having a textfield or other forms of fields as seen on typical HTML forms? This could greatly increase the flexibility of custom recipes in Dataiku.
Absolutely, you can add text fields, select parameters, etc. in custom recipes.
You can find the whole list of possible plugin parameters on this page: https://doc.dataiku.com/dss/latest/plugins/reference/params.html