One feature i'd like to see would be the ability to check the casing of track titles and prompt to automatically change those that are incorrect.
Look At The Sun would be changed to Look at the Sun or... The Sun And Me would be The Sun and Me
MediaMonkey has a similar function under scripts but i thought it would be a good idea if it were built into mp3tag.
If this were also heavily customisable like everything else is i can see it being great.
It would take a little setting up, but you could do this with a series of Replace "TITLE" actions. Make sure to check "only as whole word" and "case-sensitive comparison". Make a series of Replaces for each little word you want to replace, and you're done.
Edit: Since you wouldn't want to replace words that start song titles, set your replace strings to include a leading space.
Replace " The" with " the" would turn The Sun Over The Mountain into The Sun Over the Mountain.
Set an action for Case Conversion "TITLE": Mixed Case first. That gets everything set with initial caps. Then start doing your word substitutions.
As for words that aren't on the list, this would be a problem with a built-in tool as well. Since you asked for something customizable in your OP, that means you'd like to be able to add words to the list, right? Well, instead of adding a few words, you just get to add all of them . There's probably only ten-twelve words like this anyway. Might take you all of five minutes to set it up.
I actually went through a similar exercise about a week ago, which is why I knew your situation would work as well. I'd been using the Mixed Case feature for some time, but it started to irk me when I'd encounter tracks with Roman numerals in them. A track like "Song Title Part III" would turn into "Song Title Part Iii", which is really ugly. Since "Iii" is unlikely to occur in any real word, I added a bunch of subsitutions to the capitalization action to replace stuff like "Iii" with "III", which works great.
MP3tag is any amazing program that Florian and his crew brought to the world, and it constantly impresses me with its flexibility and extensibility.