1. Rules should help support verisimmilitude: Ideally, fire powers set things on fire. Cold powers freeze things. Lightning powers can target pools of water or puddles to extend their range. You could use fire spells on a steel door to act like heat metal. Or use lightning to hit someone around a corner who is standing on a metal floor.
Fire powers do not necessarily set things on fire. Lightning might set things on fire. Metal doors might probably not reflect lightning but instead guide the lighting into the floor. A fireball might not heat metal the way you think it does, because it does not last long enough.
We can have an unnecessary discussion about all of this. If it is in the rules, however, so that the rules dictate what the designers thought is plausible, it is hard to change that. It makes the discussion at the table more difficult. After all, it is in the rules.
So instead of solving and describing these things in a mechanical way, it is much easier to let the gaming group handle it on a narrative level.
2. More Dynamic combat (Martial as well as Casters) than 3.x. I want to see interacting with terrain, running along walls, swinging from chandaliers, knocking down pillars.....
I agree, I want all of that, too. It is all in 4E and should be transferred into the new edition.
3. More detailed out of combat mechanics, ala 3.x, as well as more out of combat abilities (ala 3.x).
I agree with this, we finally need solid out of combat mechanics.
However, 3e did not have detailed out of combat encounter resolution mechanics. 4E has the skill challenge, which does not always work.
I hope that the new edition will bring us more.
5. Don't lock me into a character class. Let me multiclass, /Every Level/ if I so choose.
6. Functional Multiclass casters! Wizard 10/Cleric 10 should be as good a character as a Wizard 20. Wizard 10/Summoner 10 as well. (Kindof like Trailblazer)
I do not think this will work. And why should somebody who tinkers with a lot of things every level be just as good as somebody who concentrates on one thing only? If you mean (since you are talking in 3e terms here) that a wizard should be different in that he can have some abilities of the cleric that shape his character and make him special, I completely agree, though.
7. Lasting Debuffs: Things like attribute damage/drain, or things that lower a stat for a decent duration (minutes), or things that change creature sizes, etc.
8. "Win Button" abilities need to be in. Things like paralysis, disabling limbs, blinding people, sleep, confusion, rage effects, etc. However! Don't limit them to select few classes. Spread them around to the other classes too, not just casters. A monk might use pressure points, etc. Maybe a fighter sucker punches someone and knocks the wind out of them.
No to the attribute/stat drain/damage. Too much bookkeeping and completely unnecessary. WotC went the right way with 4E when they got rid of that and designed debuffs around conditions and -2 etc. modifiers.
As for conditions: 4E has that for other classes other than spellcasters.
I agree it is a good thing and should be kept.
However, a "win button" sucks. The PCs will face these conditions more often, because they fight in every encounter. Whereas the opponents only face these conditions in one encounter, namely the one the PCs win. I do not think that sitting at the table doing nothing for hours because my PC is paralyzed and the spellcaster ran out of anti-paralyze-spells is a fun thing.
13. Ideally: Less of a power gap between levels. It would be nice if 20 level 1s could be evenly matched with 1 level 20.
I do not agree with this at all. I think that a 20th level character should not be challenged by 20 first level opponents on a mechanical level but instead on a narrative level by the fact that he has to decide how to use that power responsibly (or irresponsibly) when dealing with people much less powerful. That is the roleplaying experience. Mechanically he should be challenged by creatures of great power, because that is where the epic story is.