I've actually thought of a similar idea to the one mentioned above, but I agree that the mountain wouldn't be able to sustain a major retrofit, mainly because it's not really a show building so much as it is an impressive bit of set dressing around a rapids track.
Based on the space available and the design of the land, the only way I would see to feasibly do a mine cart coaster would be to retrofit parts of the Grizzly River Trail (extended queue), Redwood Forest Challenge (main queue), and Mermaid/Wine Country (the gravity building/mine part of the attraction). I personally don't see those sections of the land ever being removed in the near-term, but the performance corridor is becoming less relevant as Mermaid ages poorly and Pixar Pier encroaches on the remaining sections of Paradise Garden Park.