In the Witcher 1, there was a quest where you had to perform an autopsy. Depending on how you went about preparing, you could come to a few different conclusions, the most accurate one being the most difficult to find. Preparation consisted of making your character learn information: if you opened a book on how to perform autopsies at any time previously, the game would flag your character as having acquired that knowledge and extra dialogue options would appear during the autopsy for the player to choose. The player would still have to remember what was in the books, since the correct dialogue option would be added to a list of incorrect ones. Your reward/outcome depended on how accurate you were.
The same tests of knowledge could be used here. Perhaps it's not obviously a speechcraft tie-in, but it would produce more interesting gameplay for a speech-oriented character than the usual persuasion interface. It would encourage ingame literacy and gamify dialogue. In addition to this "knowledge-check" could be a Speechcraft check (since the right information is useless if badly presented). This could be extended from books to dialogue options, which would encourage the player to be a chatty Cathy with everyone he meets in order to collect potentially useful information.
Ultimately, this would allow for roleplaying as a sort of traveling bard, which was not possible in the vanilla game. Speechcraft there is limited to Making Friends and Influencing People, and when you think about it, has quite a cynical tone. This could be a more honorable use (but not necessarily).
- Your knowledge of magical history is tested at the Arcane University before you can advance in rank.
- Having read a bandit's note, you convince him you are his liason.
- You know how to present yourself before the Emperor after reading or hearing about the specific etiquette required.
- Having read a book on royal lineages, you deduce that a local lord is a bastard born of incest instead of a rightful successor.
- You read some stuffy books about law in order to compel a reluctant lord to bring justice to his corrupt captain.
- You collect stories from fishermen and pass yourself off as an expert angler.
- You enthrall an audience at a tavern with accurate stories (they nitpick) of far away lands.
- Learn languages (Daedric, Ayleid, etc.) and decipher artifacts or work as an interpreter
- Debates - you can choose to appeal to logic or emotion
- Have NPCs use their speechcraft skills to coerce the player (unless he has higher speechcraft)
- Track the player's disposition to NPCs and affect dialogue accordingly
- Unique face models for liars (slightly shiftier versions of vanilla faces). Could swap with a dummy NPC for the duration of the lie
- Letter-writing mechanic - With the help of some courier service, correspond with NPCs remotely (turn in quests etc.) Letter text depends on speechcraft
- Time-sensitive dialogue options (if GetSecondsPassed is realtime)
- Player can personally spread "latest rumors"