Sorry, but I not understand.

for each EP field type there is the file into /administrator/components/com_jsn/helpers/fields

I've created my own fields, specifically I changed the selectlist field to allow to search for ranges of the field. (In the search form you have 2 fields for the user-field, where you can search for example for height between 170 and 180 cm) That field inherits from the original selectlist field and the files for that are stored in a different folder and included via a plugin. That works all nice and such, but when you require all fields in the helper at that specific line, you don't check if that field-class is already loaded and if that file even exists. Since I have fields that are not inside that folder, that require fails and throws errors. It would be better to check if that class is already present and if not, then check if that file is loadable and only then include it. That would also allow to override existing fields via a plugin, simply by loading them earlier.

good observation! we had not anticipated this use. in the next release we will solve this thing.

P.s. I'm a little surprised to see that a free member have our selectlist field type...