The ENTPs I know are passionate about taking risks, working with amazing people towards a goal, starting new projects/gaining support for a concept, and taking on bigger and bigger challenges. There is nothing in particular they are consistently passionate about itself, rather they are passionate about ideas and the process of something coming to fruition.
The last healthy and fulfilled ENTP I met did venture financing for new businesses. He loved spotting potential and then using his network to bring them to life. I have also recently met an ENTP in publishing who loves covering press for new and exciting events and developments.
I think if you can find something you're good at that incorporates a lot of ongoing variety and novelty you will be able to stick to it and make it a passion. I think because it takes a lot of social/corporate climbing to get into a position where your ideas will be taken seriously a lot of ENTPs aren't happy with their careers for some time. A lot just float around from job to job until they're bored or start companies.