I agree and would love for it to happen. If Melboune can push their average home crowds to 20k it won't loose much atmosphere either. Suncorp is still pretty good when the broncs have 25 - 30k in. It also gives us the opportunity to host things like double headers and magic weekend in Melbourne in a purpose built venue and will allow for the Storm / NRL to capitalise on greater interest in finals matches.
As per my reply earlier, the foundations were built in order to support expansion - plenty of room east and west and still a bit of room to the north before it hits Olympic Boulevard, so it wouldn't be too difficult - I imagine they'd be going up more than out.
As mentioned though it's the roof that's the issue.
Indeed it is the roof that is the issue and as you say, it would require complete removal. The foundations are in place for a larger stadium as you say, but even the removal of the roof may very well make any expansion cost prohibitive for this reason. It may be a great stadium at present, but it was a stupid, short sighted design.