Suwon City in South Korea, or Pohang Steelers in the same league. Both struggle, and both would need cast improvements to succeed teams like Seongnam and Seoul. Money is something that needs to be kept an eye on, and there are rules for how many foreign players you can register for the year, so you must choose carefully.
Playing as Seoul has been the most fun save I've ever done, 3 years in and still loving it. Pohang is around third last consistently, and Suwon are always in the bottom two. There's one more team that'd be a massive challenge, but don't remember their name...