Caves and ancient ruins in fantasy media is wide-spread, so there is a lot to choose from. The first thing I thought of when I read through the text about underground ruins were the Mines of Moria from Lord of the Rings—a vast, dwarven city buried inside of a mountain. One of the most iconic rooms in the Mines is the vast, expansive hall which looks like a forest of endless, huge pillars which extend out into the darkness, so that is one of the main concepts I'll be taking inspiration from when drawing my thumbnails.
It also mentions the use of winged, female figures in artefacts. I plan to exaggerate that and use the female figure and wings in architecture, too. This may even give a divine and holy theme.
Also, the use of nature and jungles and plant life integrated with the stone buildings is a fascinating concept. Since the ruins seem to be based mostly around temples and sanctuaries, I would like to make nature the 'protagonist' in these thumbnails and final pieces, giving them a feeling of divinity and holiness to them being the 'holy artefact' of temples.
Size seems to be a key feature in these buildings—something which I can build upon to make my scenes feel and look more epic. Scale has been used a lot in fantasy art whether it be tiny or absolutely mind-bogglingly massive.