Is it possible to have a customizable priority field?

Yes, you can create a custom priority selector with one of the 15 available custom fields.

To customize your card fields, go to Settings - Card Template. Enable a custom field and change its label to, e.g., Priority level, then select the Dropdown with options type and set your preferred priority values, separating them with a semicolon, e.g.,
Ultra; Mega; High; Normal; Low

Although a custom priority field set up like this will not generate the standard priority arrows on the fronts of your cards, you can use the Card Icons power-up to assign other on-card icons to show for your custom priority levels. For example, for the “Priority level” custom field used above, you could set up Card Icons like this:

Note, that for the custom icons setup to work, it’s important not to name your custom priority field just “Priority”.

And here’s how the custom priority icons would look on the cards: