Okay, onto what I suspect will be in the revised DMG and MM:
DMG:
* Better guidelines for creating new feats
* Better guidelines for creating new prestige classes
* Cleaned up NPC stat blocks. (Did you ever notice that the cleric has both a periapt and an amulet - they share the same slot!)
* Better guidelines for creating/pricing magic items
* Better descriptions of the Spot/Listen/Surprise rules
* Some new magic items, just to make the purchase more tempting. (Hopefully, the
amulet of mighty fists will also make it into the core rules

).
MM:
* Guidelines for creating new monsters
* Guidelines for monster PCs
* ECLs for common monsters that can become PCs
* Larger font, better formatting.
* Some new monsters, just to make the purchase more tempting for those of us who have it already
Have a look at MM2 for an idea what MM1 revised may look like.
I suspect that both the DMG and MM will be not as "essential" as the upgrade of the PHB, but they will be worth it enough that I will get them.
There
are improvements that can be made to the rules and descriptions of the rules. The question is whether it will make the old books useless - I suspect not. It will be desirable to get the newer books, but given the amount of product that exists for the original version, the original version isn't going to be obsolete any time soon!
Consider that we were wondering exactly how many useful rulebooks WotC could print! Not many more, we thought. They wouldn't be able to sell many copies of a revision of the MotP, for instance, thus the revision can't be too radical as to invalidate them.
Another thing of minor import I think they might change in the PHB: Skill Focus to a +3 bonus.
Cheers!