Time Travel...would be best left alone, if only because it causes so many troubles to make useful yet not hax or to explain how it happens. The Railgun setting fudges science every now and again, but it remains fairly truthful in most cases so even locally doing a 'time stop like' effect in a localized area is difficult.

Slow down/speed up perception of things going on around you? Sure, it's kinda neat and useful.

Put objects/people in stasis for a short amount of time? Also nifty.

Full on time-stop? Problematic.

@Keveak: Check out the wiki that Shades linked to. It contains information pertaining to the setting, terminology, and groups within the setting (and other stuff!) without giving spoilers....though don't click the links in the terminology section, they do lead to the occasional spoiler.