A setting within the Number field type to auto-format it to include the % would be great.
I am very often using the CONCATENATE formula to achieve this and having this feature would save time.
I think the catch with something like this is that unlike the decimal points, once you add a symbol like % the value is no longer numeric, which can cause issues if you need to use that value later.
To suggest an alternative, maybe just allowing customizable prefixes/suffixes in form fields so we can do something like this that won't affect the underlying value:
The idea being to make it similar to how the currency field handles the $ rather than adding it to the actual text value.
I was also thinking just having a configuration option for a number field, similar to the repository fields, where you could select what display type it should be. If Percentage was selected it could just put the percent sign behind the field, similar to putting the $ in front of the currency field.
I've sent the request to the Forms team, thanks.