5) Should maneuverability return?
Yes. Did it leave? You can't have flight without maneuverability. Dragons and Djinni should fly differently.
In practice I don't really see them flying differently. I'm not saying they shouldn't be different but any existing rules I've ever seen (with the exception of niche circumstances) have them flying about the same and certainly over the party's head. Unless there is a tight hall that the dragon is trying to fly down I rarely see the issue with easing up this restriction.
8) Mundane flight in core (war balloons, gliders, airships, and planes)?
Blech. No. Too steampunk. Save it for some expansion down the road.
8) Mundane flight in core (war balloons, gliders, airships, and planes)?
Core as in assumed? No. Core as in the main three books? Yes. It's not usually my cup of tea, but it could lead to some very interesting stuff for some people, and it's not at of the realm of fantasy to have dirigibles or the like.
I like what JamesonCourage says here. It isn't my cup of tea but I wouldn't say that it is outside the realm of fantasy to have those things.
I remember seeing lazer cannons and handguns and whatnot in the 3.5 DMG. I never ended up using them, but if I had ever set my DnD game in the present or future settings then I'm sure those things would have been useful
right in core.
3) What is your preferred balancing factor for Fly spell (control, duration, speed, maneuverability, action economy)?
Mainly action economy, but with a sprinkling of speed and duration. Make any ranged attack (spell or otherwise) take a full-round action (the hated full-round!) instead of a standard, and make it take at least a move action to stay in the air. That way, you can't do both (fly and make any ranged attack) unless you circumvent the restriction (Haste, special ability, feat, etc.).
I don't really want the restrictions on flight to be more spells. Especially when you consider that it is the fighter that loses out by not having Fly nor Haste.
If anything I would like flight to be a basic spell, preferably lower level. Then have higher forms (or combinations of spells) give different tricks instead of removing drawbacks.
To answer the OP.
Q1: When do I want it to show up?
A1: Low level, but with lots of drawbacks.
Q2: When do I want to see items (suggested)?
A2: Whenever I choose? I'm okay with level 10 (or even lower), as long as the solution is
never "you pretty much need fly to beat this".
Q3: Balancing factor.
A3: Ah, yes this is what I wanted. For me the best balancing factors are a little of all the choices given. But with emphasis on action economy and duration. (I would also throw in a concentration component.)
If there is a limited number of things that you can do while flying while enjoying being able to fly next turn then I'll be happy.
If there is a fixed duration, so you aren't flying ALL the time, then I'll be happier.
If you have to spend a bit of effort, or can be otherwise knocked out of flight fairly simply then I'll be ecstatic. But I think you need a bit of all three.
I play 3.5 and the problems I partially see (but others seem to experience in spades) are as follows.
It assumes that you have flight to defeat some opponents but it doesn't balance things when those opponents don't have flight - like melee or grunts without flying.
It forces everyone to have flight because flight can override so many problems. Ie. climb checks, balance checks.
It is an automatic avoid all for casters with the right combination - usually when added with missile deflection and some form of greater invisibility.
It it always on. So you can just assume flying as a standard.
Now, if you put limits on it, like I put "landing" means it ends, then you can stop a lot of those minor problems. If they have to concentrate (meaningfully) or lose actions while flying then they'll use it sparingly.
Of course this only helps if flight is kept as a vancian spell, which they can or will run out of uses for. If not then I see no easy ways of balancing it.
Q4: Drawbacks?
A4: See above. I hope that applies.
Q5: Maneuverability?
A5: As I said earlier, I understand the need for it but I haven't seen very good application of it. I would prefer just to have a simplier form, with perhaps 3 forms of maneuverability. Then define one as not being able to do tight corners, and one as being able to fly perfectly straight up. IMO then have just terrible/poor, good/average, and perfect forms - I don't need anything else.
Q6: DMG Info.
A6: See 5. I need info on strong winds, or the other aspects of what happens when you are flying, but the flight aspect should be relatively easy to deal with. I would like the "in sky" equivalent of the "dungeon" or "city" terrain features.
Q7: Tactics
A7: Yes, certainly. Enough creatures fly that it would help if we knew the best ways of dealing with them. Even if it is just simple conversions of what a ground maneuver would look like against a sky creature, but preferably some minor instructions on how ground creatures can fight flying ones. I also want similar manterial dealing with burrowing creatures.
And Yes. I want it because I plan on letting my players fly at some point, and yes I want it because they're going to be fighting dragons and the "jumping off steeple" approach isn't always going to work. All that being said I do want tactics that work, because it is at least a minor part of the game and people who haven't played for 30 years may need some help.
Q8: War Balloons?
A8: Yes, sure. See above reply.
Air balloons can be cool, and once again, just because they haven't shown up in my game doesn't mean that I shouldn't have rules if I do decide to have them show up. In my setting I've had (and planned) nearly every kind of micosetting I can imagine, including a sky-realm. It just depends on if the party encounters it or not. I especially want rules on those kinds of things (just like I want rules for regular ships in the water [or below]) because trying to MAKE them is a pain. I know, I've tried. Having to come up with stuff like that from scratch is annoying to try and do correctly. I don't have a design team to come up with these ideas and I appreciate when my game does it for me.
--I couldn't care less what is 'Core' or what isn't as long as I get to play the game I want. So I ignored that part of the question.