I can mod Singleplayer maps for Halo 2 Vista, just PM me with your request, I can edit weapons, your character, vehicles.
Although you must remember the potential is limited since I can't add anything that wasn't in the map already but I've added things like pistols the shoot like a gauss warthog and a needler that shoots like a phantom.

Although I must say my mod of the level "oracle" made the ending extremely hard even on Easy since the plasma rifle shoots like a beam rifle and even on easy just a few hits you're dead. On Legendary one hit you're dead. But anyways
I can do some extreme modding like a while back I made a map for a friend, it was Delta Halo but you and your allies were covenant while the enemies were humans. Twas fun, the program I use to mod is Gravemind 1.5.