A few thoughts occur to me.
Slice60 is near perfect for what it does. But it's command line. There was a gui made that set up the switches and together they do the job splendidly.
<center><img src="http://mtm2.com/~forum/images/turboSliceGui.gif" width="393"></center>
I'm not saying you need to copy that exactly. However, it should serve as a source of ideas.
Also, there was a program called GreenGiant's Axe.
<center><img src="http://mtm2.com/~forum/images/GreenGiantsAxeGui.gif" width="281"></center>
It, like traxx, has a preview window that shows lines where the slices will take place. There are two problems with the axe. First is that it does not handle the overlap (it was made for mtm1) and second it no longer runs on the new operating systems (the fate of many older programs). Still, it was handy.
Then, of course, there is traxx.
<center><img src="http://cownap.com/~mtmg/traxx/images/cust08.gif" width="378"></center>
The main problems with traxx is that it does not do the 256x256 overlap properly, and it does not swap the outer edges (on any size), and you're constrained to 16x16 squares maximum. The good things about it are that you have the option to do the overlap or not, you can move the slice grid so an image doesn't need to be a perfect fit or you can slice a part of the image, and, of course, the preview.
An option to prompt for file overwrites is sometimes handy. But low priority.
Would it be possible to detect files that are exactly the same? How might they be flagged? When we're concerned about texture memory, removing every redundancy will help. Would that be built into the slicer, or would it be better as a stand alone that could analyze a directory's content?
In sum, slicing 60 and 240 base textures are the primary goal.
Having thought about it, slicing 64 and 256 base textures without overlapping - just a straight out cut - might still be desirable (for instance, for models or ground boxes or mtm1).
Slice60's stretching feature (ie no overlap) has merit although I don't know how you'd work that with 240 textures.
If anything else comes to mind, I'll holler ;-)