Daaammnnn... you really thought this out didn't you? You have some really cool ideas though, particularly about the Spiker. I was just thinking about it today; at the moment, it is useless. It just has so many problems. The Spikes are VERY slow, they fall over time, and it is inaccurate. To fix the Spiker, I would drastically increase the speed of the Spikes, and remove the irritating thing where the Spikes fall. And I would turn the damage up a bit. The Spiker sticks out as a perfect example of what could have been a really awesome weapon. Bungie just nerfed it for some reason. It's worse than the Halo 1 Needler.
I would also like a remote detonated explosive. Attach it to a mongoose and drive into a person and boom! This is always fun.
Wow, you literally read my mind! I was going to mention the custom creation but I thought that would be beyond the modable range, i don't no anything about modding but doing something like that's seems difficult. If that was possible, i would keep the "areas" that can be set to a hill or territory or whatever, and have their own trails when in the area. Then in customs, there would be a multi-gametype, a really big score possibilities, and you could turn each gametype on or off. "allowing minigames to rule the world!!!?!?" - it might get more out of hand, it may turn galactic. This would be amazing, and bungie should cerealusly con cider it.
Hmm... I guess if i was allowed to mod the game i would introduce Pelicans, in forge and custom games as a few others said. Would be pretty epic to drive one around Avalanche. I'd also perhaps make a 'heavy' version, with rockets or something.
Probably changing map scenario's. I'd love to have a legit snow version of Valhalla, or Sandtrap (Snowtrap?). I think it'd be awesome to change the theme of maps.
If I could mod Halo 3 I would make selectable colors for projectiles/grenades. So you can change the Splaser to fire a green laser, or have blue needles in the needler, or a red plasma grenade. Imagine Rave with this! Multi color needles and stickies everywhere would be epic. But it would only be allowed in customs, because it would get too annoying in MM and it would give away who is who. I would also give the Warthog hydraulics.
Full custody of terrain and object editing of all maps. Can't believe it hasn't been said... Other things too, but I'm good with just that. Oh wait, rescope the magnums and increase power slightly. That way they would get used...