Mickey Mouse will never enter the public domain. Disney will always get a retroactive extension to copyright that includes it through congress before that happens. You can argue that this is bad/unfair/unconstitutional/great/etc but that is the practical reality. Any practical proposal for a reform of the copyright system has to take this into account.
Under the current system this means that anything from that era forward also stays out of the public domain forever, including “orphan works”. The loss of these “orphan works” that are long out of print and with no clear owner is the one thing that (almost) everyone can agree is a Bad Thing. So, any reform that has any chance of passing must improve the situation with those items while preserving the interests of politically powerful copyright holders (Disney, Sony, etc). I can see some options that could do it, though in any real world implementations would reveal some flaws.
Opt-in renewal is the one we hear tossed around the most. This sets forgotten material into the public domain but preserves the copyright on anything the owner finds worthy of a nominal renewal fee. It has the added bonus of registering who you can license the publishing rights from.
Letting works fall into the public domain after being out of print for X years could also accomplish the above goals. It would also have the added bonus of encouraging publishers to keep a work available/in-print to preserve their rights.
Lastly we could instead create a special extension for trademarks, franchises, and corporate identifiers. Things strongly associated with an ongoing business could be protected for a longer period. DC would keep Superman and Disney would keep Mickey, but an out of print science article would not get the new extension and would eventually enter the public domain.
None of these proposals would satisfy copyleft purists or “hands off my copyright!” paranoids but they could be a reasonable starting point for compromise.