The Pantheon: The ancient building still being used after 2,000 years

There are also many amphitheatres still around and often in use (concerts and theatre performance) all over Europe (and beyond): https://en.wikipedia.org/wiki/List_of_Roman_amphitheatres

Isn't it right next to a highway?

Since it’s made of unreinforced concrete, getting it permitted in almost any jurisdiction in the world would be exceedingly difficult. Which is somewhat ironic, as it’s the lack of corrodable reinforcing that’s enabled it’s extremely long lifespan.

Stonehenge is approx 250 metres from the A303, which is one of the main routes into the South West UK. The A303 at the closest point is a single lane in each direction, and frequently very slow at that point, so isn't massively obtrusive in the sense that a 6-lane motorway would be. That said, there is a plan to dig a tunnel nearby to get rid of the road completely.

Stonehenge is debatable as a visitor attraction, especially if you are in the UK only for a few days. By coach from London it is pretty much a full day, when there is a lot more history in London itself. In my opinion. Stonehenge itself (as opposed to the larger archaeological landscape) is small compared with Roman or Greek remains, so if you don't have an appreciation of the history of the place, may be underwhelming. It also gets crowded very quickly, although the fact that a close approach to the stones isn't usually possible means that everyone gets a view. Last time I went I arrived early, and got some great HDR images of the stones under an impressive moody sky.

[Edit - I also agree about Avebury being worth a visit. You can walk amongst the stones themselves, which are almost part of a historically pretty village. The really impressive structures in the nearby vicinity in my opinion are the hill forts such as Old Sarum [0] near Salisbury and Maiden Castle [1] near Dorchester. These are truly massive Iron Age earthworks.

[0] https://en.wikipedia.org/wiki/Old_Sarum#Prehistory

[1] https://en.wikipedia.org/wiki/Maiden_Castle,_Dorset

And you didn't even mention the tacky "King of Italy" coffins.

I was discussing this yesterday with friends: the building is still a church but a few years ago it was sold to the State (so the Church doesn't have to be responsible for the upkeep...). This means the State could, in theory, throw out all the Catholic paraphernalia and ripristinate a classical-looking interior, if local government so wished. We don't have images of the original Roman statues but I'm sure there are plenty that could be moved in from other locations. It would be definitely something that would stir international attention and attract more funds (always good) and more tourists (not always good - it's debatable whether one of the most-visited hotspots in the city actually needs more attendance...)

In any case, it will likely never happen; not only because of the massive outcry from religious quarters in "the" Catholic city, but also because a lot of the academic world is strongly opposed to this sort of "restauration" both on philosophical and practical grounds, after some massive missteps that lost us priceless artefacts in the previous centuries.

It's a shame, I think a classical Pantheon would be superb, but that's life.