Welcome to the community! Unless a mod says otherwise, I'd say you've posted this in the right place.
1. I like that powerup idea! I tend to forget to add custom powerup options in the gametype itself, and then when I use custom powerups, they do nothing in-game.
As for generating lighting, it would be a nice option as well, although it generates lighting every time you switch to player mode to give you an idea of how your map looks while you're building it. Then, if something looks off, you can fix it before your map gets too complicated.
2. MCC scrapped the budget and uses an object limit instead. However, each type of piece is limited as well, so you can't drop 200 Kill Balls in your map and call it a day. It all depends on how much strain an object puts on the engine. (This is typically the part where Twin sheds some of his Blam engine knowledge)
3. Yes. All of my yes. Covenant textures would be a bit tricky, however, as their architecture is very smooth and rounded and would look awkward on blocks. Curves aren't easy to deal with. Forerunner architecture tends to be more sharp and triangular, which would be easier to work with.
4. I'd say this one is most likely to be in the game in regards to brand new features. Even if the number of forge canvases is limited, it would give each one a huge amount of replay value. However, I imagine that different weather effects could put a lot of stress on the engine, especially when playing online.
5. Animals, yes. I'd say that's a possibility (so long as they're passive and not aggressive like Guta), especially since CoD's "revolutionary" fish AI was first featured in Super Mario 64 almost 20 years ago.
AI enemies, however, are a different story. You'd need to give them the ability to find paths, which is extremely difficult, if not impossible, for the average Joe. Stationary AI enemies, like those Promethean Turrets in Halo 4, would be much easier to implement.
6. Vegetation would be fairly easy to add in. Water, however, could be tricky to work with. If a map were to be submerged, you'd need to alter the player physics. If it were to follow fluid mechanics, that could be extremely taxing on the engine. Xbox One is a very powerful system, but the game engine itself and online play hinder its abilities.
Those last two are developer level tools. It'd be rather tricky to implement them in a way that is user-friendly. Of those two, however, terrain customization seems fairly likely to show up in a future Halo game.
With all that being said, it's far too late in the Halo 5 development process to implement brand new features. However, maybe some (perhaps all if we're lucky!) will appear in forge. We'll find out when it's released on October 27th!
Hope to see you around in the forge!