Idealism is the view that reality is, in some sense, fundamentally…mental. But to get into the Ideal Idealist Club, you have to hold a very special version of that thesis. It's not enough to say that mind is more real than matter—you have to see mind as ultimately eternal and unchanging. Even Berkeley, one of the all-time great anti-materialists, doesn't qualify by that standard. Only the few, the brave, the Truly Ideal Philosophers can gain admittance into this exclusive club.
This group would never have existed were it not for the Father of Western Philosophy. Who else would have been crazy enough to introduce the notion that somewhere there is an invisible ideal couch that is more real and more perfect than the imperfect material couches we see in the world around us? Answer: no one. Thanks, Plato. We couldn't have done it without you.
Ideal Western President
Well, he's not Plato, but he's the next closest thing. Everything ultimately emanates from some ideal transcendent source? Check. That ideal source is eternal and unchanging? Check. Matter is just a very imperfect imitation of the true non-material reality? Check. We think we can safely say that Plotinus is a very good imitation of the original.
Resident Ideal Buddhist
Vasubandhu definitely can't be counted as an explicit follower of Plato. In fact, it's highly unlike that he had ever heard of Plato, living in India in the 4th century CE as he did. Instead, as one of the co-founders (along with Asanga) of the Yogacara or Mind-Only School, he presents us with a distinctly Buddhist version of idealism.
Anyhow, the name says it all—or at least an important part of it: "mind-only" means that only the mind exists. Yes, you think there is matter; you think there is an external world out there. But these thoughts are all just projections, in some sense, of the mind.
Now Vasubhandu doesn't deny that we die or that many aspects of our consciousness don't survive the death of the body. But he does posit the enduring existence of the alaya vijnana or "store-house consciousness." This most basic mode of consciousness contains the traces of all we have ever done and continues from one lifetime to the next, endlessly. It's true that Vasubhandhu might not exactly say that the alaya vijnana is unchanging, but we won't worry too much about such details. Welcome to the club, Brother.
Resident Ideal Hindu
Here's another guy writing in blessed ignorance of Plato. Ol' Adi lived in the 8th century CE in India. He's one of the main expositors of Advaita Vedanta, one of the (many) Hindu philosophical traditions. The key idea here is the ultimate identification of atman, the individual soul, and the universal (and, yes, eternal and unchanging) spirit, Brahman. That's right: we are the world.
Plato needed a little bit of convincing before he realized that Shankara was a kindred spirit. But in the end, he welcomed him with open arms.