There are many elements that go into creating a travel site. Off the top of my head here are a few of them:
1. And # one in importance, what can your site offer that you can do better than competitors or that competitors don't offer?
2. Will your visitors' focus be on:
- method of travel
3. Is your demographic
- Local commuter travel
- Regional travel
- National travel
- International travel
4.Will the majority of your visitors be traveling for business or pleasure? This involves time of year for pleasure travelers, but not so much for those who travel for business.
If you already have a travel forum, the most popular topics entered by your visitors is a great place to start your research.
Finally, check the competition and see what their focus is on and what they do best. Here's a link to popular travel sites: http://www.ebizmba.com/articles/travel-websites