Essentially, this depends on what elements the game has (and how much additional coding/eventing would be required). You could have skills for:
- Traveling: I mean large-scale traveling with this one, on world maps or even automated traveling. For instance, you could have Pathfinding skill that reveals additional places on the world map as you come along (secret cavern entrances etc.)
- Crafting: allowing for higher quality items to be crafted
- Dialogues: There's room for many skills here. You could allow the player to persuade other characters, intimidate them, tell if they are lying/hiding a secret, make them tell you additional information and so on. Maybe you cannot even talk to the elves at first because you simply don't speak their language. Those could even be used in automated dialogues, which would then branch off depending on the respective skills. Since I come from the western school of RPGs, which are often quite talkative, dialogue skills are some favorites of mine, even though they usually take a lot of work.
- Environment Interaction: This is where I would count most of cekobico's skills to. The opportunities are endless, but all of them require additional eventing. Some of my favorites:
- Languages: Not necessarily for "secret" hints, but I think it adds a lot of atmosphere if the heroes find an old book in the wizard's tower which actually requires knowledge of an old language to read. In this case, the player knows what is lacking and can come back later on with the required skills. (My Temple of Eternity, which is too huge anyway, has something like six additional language skills, although only one of them is required for the main campaign. But if you really want to open the entrance to the crypts of the ancient serpent wizards, you have to speak their language. Hissssss.)
- Pick Locks: For locked chests and doors.
- Detect / Disarm Traps: Obvious...
- Spot Secret Doors: Allows for either hidden treasure rooms or for alternate, less dangerous passages.
- Herbalism: Allows you to gain more herbs (usually for crafting) when picking some up from an event.
That's all I can think of right now. But in general, whenever there's a certain major game element, there's the potential for skills. A game with lots of unicorns could easily have skills which enable you to catch and ride unicorns. The one thing I have learned
not to do is including skills which are not really relevant in the game, just because they sound cool to have.