I think whilst our owners obviously could build a brand new stadium, I doubt they will be anywhere near considering that. The stadium just needs to get to 70,000 to be able to host CL finals. From a financial viewpoint that could make sense but trying to get to 80k, 90k, 100k wouldn't as the additional seats would only be low-value/hard-to-sell, poor views. The only event worth bidding for to host in such a large stadium is the WC final and if that's in the UK it's not going to happen anywhere other than London.
So the question is can the Etihad get to 70k? Possibly, but it might need a change in legislation on safe standing so it allows more fans to stand in some blocks than currently, in conjunction with new roofs and extended third tiers in the east and west stands.
That's my view, anyway. I suppose if the cost/practicality of doing the sides is more trouble/cost than just building a new stadium I could be wrong, but I think that's unlikely.