There were two plans: a Swales plan which was detailed in several match programmes in 1981/82, and a later Lee plan in 1994/5.
The new Main Stand roof in the summer of 1982 was meant to be the start of the Swales plan, which would see the other stands developed over the next few years. IIRC, capacity was intended to drop to 48K (32K seated and 16K standing) from 52K (26K seats and 26K standing). It was supposed to be ringfenced Development Association funding, so it wouldn't affect the financial position of the club itself, and IIRC the first phase cost GBP 1 million.
However, whatever the funding source was, it obviously dried up, because that work on the Main Stand was the last we heard of it. No one ever referred to the plans, either: it was as if they'd never existed.
When the Kippax was built, IIRC Lee basically came up with a plan to make the capacity 45K eventually by putting extra tiers on the North and Platt Lane stands, filling in the corners and rebuilding the Main Stand. The capacity of just over 32K that we had with the new Kippax in 1995 was generally regarded as inadequate for the club if we were ever to progress. I'm not sure if this was ever publicly announced (I didn't think it had been in the match prog, but that may just be my memory not being what it once was). However, there were certainly, as levets says, plans doing the rounds, and I remember King of the Kippax did a feature on it using very rough illustrations they produced themselves.
These were officially not taken forward while the club examined the possibility of a move to the new Commonwealth Stadium once Manchester won the right to host the 2002 Games. However, there would also have been considerable opposition from residents to further expansion, while the club's financial position was hardly conducive to carrying out a large scale remodelling of the stadium.