I'm not too happy with the current DMG. If I were to re-issue the current DMG, this is what I would change:
Chapter #1: No change
Chapter #2: In my new DMG, this chapter would comprise about half the book. There would be a section of every skill with a 2-10 examples of different possible DCs for various tasks that might come up in an adventure. It would also contain a section on ambiguous situations and what skill to use, e.g.: what perception skill to roll on if a character could pick something up by smell, like a forest fire, or principles for substituting Jump, Tumble and Balance for one another in different physically complex combat situations.
Chapter #3: The section dealing with wilderness adventuring would be vastly expanded. Instead of giving 1-2 pages per ecosystem type, 10-20 pages would be more appropriate and would include customized weather tables to address penalties and bonuses due to weather in each terrain/climate type. It would also explore more fully problems of wilderness survival, suggesting reasonable intervals between getting food and water and the consequences for not getting these things. It would also cover issues like sleeping in bad weather and required protection.
Chapter #4: Perhaps the 18 pages currently spent on those large tables of NPCs one can use in a hurry could be cut down or eliminated entirely. Or better still, moved to an appendix of the Monster Manual or included in the Human section thereof.
Chapter #5: No change.
Chapter #6: This chapter does not belong in the DMG and putting it in here just makes players feel they have to purchase it. While the various character generation methods are appropriate to a DMG, Prestige Classes, Epic Levels and the sections on familiars, cohorts, mounts and animal companions all belong in the PHB.
Chapter #7: This section could be considerably paired-down. Rather than give us 72 pages of magic item listings, perhaps the authors could have spelled-out a systematization for all magic items, instead of just systematizing weapons, armour, wands, scrolls and potions, still allowing for the creation of strange and unique wondrous items but within a framework DMs could use and understand. Tailoring magic items is a big part of developing a coherent campaign feel. Why is it that the DMG give instructions on creating classes, creating races, creating spells and not on creating magic items (not the current section that just purports, falsely, to do so)?
New Chapter: I think there should be a new chapter in the DMG to cover common misperceptions about rules, conflicts over their meaning and general principles for resolving situations where the rules are ambiguous.
New Chapter: Mass battles. I'm not saying that the DMG should provide so much data as to eliminate the need for supplements on this issue but I think it would be helpful for the DM to have some general rules for what happens around the characters if the party becomes involved in a larger battle, for instance, if they arm and lead a band of oppressed villagers against their oppressors. I'm not talking about a battle of thousands but some shortcuts so the DM can predict how the battle will go between two groups and the interactions between the part of the battle the characters are in and the adjacent parts.
In my view, the job of a DMG is to provide DMs with a resource that helps them frame situations from their imagination in game terms. I'm not really interested in the DMG being used as a supplement for "advanced" players/characters.
All this stated, there is no need for a DMG II. I think about 75 pages of the current DMG could be eliminated with no harm done.