Because you're hardly going to attend Whitby/DV8/WGT/Under the Blue Moon/insert name of Gothy fest here without a suitcase as unique as the rest of your garb (or at the very least, black).
I wouldn't want to offer any hard-and-fast suitcase-customising rules (that would be a little odd) so if I may I'll offer some general tips (and some pictures of my own suitcase).
You are likely to need:
- a suitcase, obviously
- fabric paint
- fabric glue
- patches (handmade or otherwise)
- appliques (ditto)
- badges (although these can be a right pain on a crowded train as they tend to get caught on things)
- miscellaneous other items (such as stick-on jewels, plastic spiders, safety-pins, lace, etc.)
Be aware that, if travelling abroad, you stand next to no chance of getting a suitcase covered in badges/pins/other sharp objects onto a plane.
The plastic spider on the handle of my suitcase required a very strong all-purpose adhesive - everything else was attached with fabric glue (make sure it's waterproof!).
Scraps of fabric can be used to make interesting details... the stripy accents and hearts on my suitcase are from an old sock.
Keep an eye out in antique shops for vintage luggage. You could leave it as-is for a traditional look or give it a steampunk theme with cogs and gears.