So far all but one of the riddles I saw in the game were about language (one was a logical one). I think the language-related riddles shouldn't take such a large portion, because they're incomparably harder to solve for non-native speakers, which make up a large portion, if not the majority of the player base. Also you should really take care that there is only one solution to your riddle, or accept all possible solutions. Here's an example for a riddle that has at least three solutions (I'm only giving solutions that are not accepted, so I guess that doesn't count as spoiler)
[NPC name removed] had a chest that was just a little too heavy for him to carry. He added something to the chest and then he could carry it. What was it he added to the chest?
Answer 1) Hair (To put hair on your chest = to strengthen you up, to make you a man)
Answer 2) nut (chest+nut = chestnut, which is a much lighter item)