one of the oldest, and widely used parser function is #switch
when a template uses a parameter as feeder to #switch, templatedata should be able to list the "legal" values, and also allow indication whether the #switch expression has #default clause, so a wizard will be able to provide a drop-down with the values, and optionally make it a "drop down combobox" which allows the user to feed a value not in the list.
if such syntax already exists, and it's only my ignorance speaking, please teach me. otherwise, please add switch property (call it whatever - html uses "select" and "options" i think), e.g.
"params": { "bla": { "description": "bla bla bla", "switch": ["preset value 1", "preset value 2" ...], "other": true } }
peace.