(Note: originally intended as a short reference, turned into a longish thinking exercise.)
Comic World News has a nice interview with manga editor Katherine Dacey-Tsuei about the famous four-woman mangaka collective CLAMP. Especially notable for me is the short discussion about CLAMP's creating 'crossovers' by letting characters from one of their series make cameos in others. The practice of letting characters surface outside their own series is far from rare in manga, or indeed in comics or strips. But Dacey-Tsuei points out that whereas this is often simply a ploy to increase sales, CLAMP gives it a different twist by creating 'alternate universes' for the characters, giving them a different fate than in their original series.