The original PS1 came out on Dec 3, 1994. For the 20th anniversary Sony announced this Limited Edition Console Bundle and Controller (That could be bought separately).
With that being how they celebrated the milestone (more or less).The obvious answer for the 25th Anniversary would be to release a new Limited Edition Console. However, I wonder if the earlier talk of PS5 being about "History" might lead us to something more interesting for the 25th Anniversary. Such as a Anniversary Sale similar to the Days of Play event earlier in the year (Where we can see a multitude of Classic Titles on Sale).
If the future of Playstation is about the history of the system, what better way to start than on the 25th anniversary. While an actual PS5 reveal might be too soon, A state of play focusing on classic titles (There are those rumors of a Demon's Souls Remake) would be interesting as well.
Typically Anniversaries aren't treated all that well if they don't fit within a FY plan, but I'd love to see the 25th celebrated with a bit more oomph. Even something like the PS Classic commercial that perhaps entails numerous fan favorites from all of their consoles would be quite the treat.