There isn't "one Indian metaphysics" but many schools (with 3 of them being the most prominent - Advaita, Vishishtadvaita and Dvaita) called the Vedanta schools. These schools deal with answering fundamental questions:

1. Who am I? What is I? Is the concept of "I" an Illusion or is it Real?

2. What is Consciousness and its nature?

3. What is Karma?

4. Is there a God?

5. If there is a God, then what is the relationship between God and I? Am I different from God or the same as God?

6. What is Creation? Is it an Illusion or is it Real? How big is it? What are the various planes of existence (also called Lokas)?

7. What is the relationship between God and Creation? Is it dependent, independent or one with God?

8. What is the relationship between Creation and I?

9. What is the ultimate purpose of existence? Is there a life beyond this life?

10. What is Time? Is it Relativistic or Absolute? What is the Age of Creation? Is Creation cyclical or is it linear? How does Karma tie into this?

11. What is considered Pramana (Proof) for arriving at an answer for these fundamental questions?

> an example of those groundbreaking Indian metaphysics studies

It is hard to point to one because Indian metaphysics is vast. But for me, the most groundbreaking Indian metaphysics study is in coming up with a value for the Age of the Creation by dividing Time into Yuga cycles.

To quote Carl Sagan "The Hindu religion is the only one of the world’s great faiths dedicated to the idea that the Cosmos itself undergoes an immense, indeed an infinite, number of deaths and rebirths. It is the only religion in which the time scales correspond to those of modern scientific cosmology. Its cycles run from our ordinary day and night to a day and night of Brahma, 8.64 billion years long. Longer than the age of the Earth or the Sun and about half the time since the Big Bang.".

Even though he got the part about Creation undergoing continuous Creation and Destruction right, the figure he quoted was wrong. The Multiverse, as per Sanatan Dharma, has a lifespan of 311.04 trillion years with our Universe being a really, really tiny portion of it (also called Brahmanda. There are infinite Brahmandas in the Creation and the Creation itself is divided into 14 planes of existence / parallel universes called Lokas). The day and night of Brahma, as per Sanatan Dharma, does not constitute the length of the Cosmos. So 8.64 billion years is definitely the Time duration of a day and night of Brahma but it does not equate to Age of the Creation. The Age of the Creation = 100 Brahma Years = 311.04 trillion human years. And yes we are talking relativistic Time here.

Coming to relativistic Time, I really like how such concepts are so well explained through stories in Dharmic scriptures. One of those is about a King named Kakudmi who took his daughter Revati to visit Brahma Loka (another plane of existence / parallel Universe) which is the residence of Lord Brahma (the deity incharge of Creation in Sanatan Dharma). He felt there was no one worthy of her intellect and beauty on Earth and wanted to consult with Brahma to get her a suitable husband. He nevertheless took with him a shortlist of candidates which he reluctantly prepared to be presented to Brahma. One arrival in Brahma Loka, Brahma was engaged in listening to musical performances by the Gandharvas (artists in the Dharmic lore). Kakudmi bowed and waited patiently. Once the performance was over, he approached Brahma with his problem and list of suitable candidates. Brahma laughed loudly and explained to Kakudmi that Time runs differently in different planes of existence and that in the short time they had to wait in Brahma Loka to see him, 27 Chaturyugas had elapsed on Earth. 27 Chaturyugas. Each Yuga cycle = 4,320,000 human years. So 27 Chaturyugas = 116,640,000 human years.

“O King, all those whom you may have decided within the core of your heart to accept as your son-in-law have died in the course of time. Twenty-seven catur-yugas have already passed. Those upon whom you may have already decided are now gone, and so are their sons, grandsons and other descendants. You cannot even hear about their names. You must therefore bestow your daughter upon some other husband, for you are now alone, and your friends, your ministers, servants, wives, kinsmen, armies, and treasures, have long since been swept away by the hand of Time.”

Brahma then comforts King Kakudmi and prophesizes that by the time he reaches Earth, God would incarnate and with him would incarnate various Devatas (demigods) and one of them, by the name of "Balarama", would become her husband.