Perhaps you should give the whole sentences as examples when asking about the verb aspects.

(a) I can only imagine something like this for your example: Я хочу часто приезжать сюда = I want to come here often. That’s a repeated action, and that’s the reason to use imperfective. Btw, приехать means to come, not to leave. You mean present time, but you could also say Я хотел часто приезжать сюда (past) or Я буду хотеть часто приезжать сюда (future). Time reference doesn’t make difference. Your considering the action as repeted one is essential. Sadly, I can make up no sentence with your exact example about a single action.

(b) Я хочу приехать сюда к обеду = I want to come here by dinnertime. Yes, you mean future time, but, the same, your referring to the future itself doesn’t determine the use of the perfective here. You may as well say Я хотел приехать сюда к обеду (past). While saying that you didn’t mean that you wanted to be in the process of arriving. You meant you wanted to arrive there and that you will be present there as a result of your arriving, the action of arriving is implied to become complited. And such a thinking is the reason of the use of the perfective.

Again, you should give the whole sentense while asking a question so that the context will be obvious.