A couple of main suggestions, both related to each other, with others afterward.
First, change the different steps into dynamic functions. What I mean by this is to make it so that the first step, instead of being 'forced' to be regex, it would be the first function and could happen to be regex, or replace, keep, etc.
Next, allow adding/removing of functions as well as the ordering of them. The same functions can be added multiple times but would be independent of each other.
It may be a bit of a chunk to bite off to do things this way but I'm sure you can see that the benefits would be well worth the effort. For example, a cleaner interface. If I only need 4 or 5 steps total, then I only have those functions showing and it looks less 'busy'. Another example, when using the same function multiple times, I could have several RegEx's for example, that might all perform a similar function but with different results depending on if there is a match or not.
Another suggestion is that for the RegEx, allow the user to use 'raw mode', where they would use their own delimiters, including modifiers. I needed to include (?i) to make it case insensitive, for example. I could use #regex#i, which looks cleaner and also lets me use # instead of / as a delimiter, making it better for me when making the regex if I need to use a different one than the usual.
For the file listings, it would be nice if I could choose which fields are visible. I know I can resize a field to 'not show' but it's still there and if I need to resize another field, I might 'unhide' another instead.
New function: Re-date/time function. I suppose the easiest way for this to work and to explain would be like this. Using regex, I would set it to 'capture' the proper date/time I want to update. Next, I would enter in the captures in boxes asking for year, month, etc. After that, the 'timezone' that it's currently in and what I want it changed to (along with an option to honor DST). Finally, the format I'd like it returned in (something like %Y for 4 digit year for example). Something like that would be immensely helpful.
New function: Substitution. Where there are multiple 'finds' to a single replace. The finds could be regular text or regex format, and would allow someone to handle misspellings or such to be changed to a 'correct' or desired result. Once a match is found, the rest of the tests would be skipped, which is what would make this different from using multiple regex functions to achieve the same result. I'm not sure how many people this would benefit, so it'd be understandable if this one sat on the side until there was a demand for it.