I predict so many players online at once for at least the first week, I'd stick to things that are not lessened by the presence of large amounts of players. By this I mean things such as:
1. Fishing
2. Mining (pure ess only, most rocks way too crowded)
3. Fletching (a bit later, alchers are going to need thousands of yew longbows and I promise there will be more people selling bowstring and yew logs than people selling yew longbows so chance here for lots of potential profit for a committed fletcher)
4. Combat in general (if you are committed you can easily get higher enough combat to be killing NPC's most people can't, if you focus on combat only you could easily hit the uncrowded level of combat farming gold and xp freely)
5. Runecrafting (people need runes and most mages won't want to craft all their own runes, too slow)
6. Thieving (pickpocketing isn't playercount limited and pure gold or other valuable items such as gems will be important for a growing economy)
That's all I can think of right off the top of my head, I'm sure other resource gatherers will make plenty but those are the skills that won't be connection speed or time of day dependent so the most reliable.
