When creating an intro/ outro of a track it sometimes help to make use of ambient sounds for a couple of reasons to fill up the frequency spectrum, to give the track some character and build up excitement, or to create a minimum sound reference level. To do this you can use anything actually. Common things like white noise, stabs with lots of reverb on it, sample pack fx's or your actual theme sound but then put in a filter with some cool effects on it etc. Always mix in these sounds at a soft volume. You barely need to hear them in your mix but if you take them away you are missing something...