Kaiserreich's Left Kuomintang game-play is the gold standard against which all other HOI4 mods, demonstrably leftist or otherwise must be judged. It is engaging, reasonably ideologically coherent, uplifting(or not if you pursue one of the less savory paths), full of different choices that will impact the long term dynamics and where your new government ends up, tactically and strategically engaging with lots of different ways to engage in wars and alliances and perfectly utilizes the new HOI4 government power struggle mechanics.