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A few revisions that seem more applicable to how the game developed:
The "Weapon Shop/Armory" would be better manifested as Loadouts unlocked by getting a certain number of points in the "Campaign Scoring" Style for each specific person.
The same way new Tiers are unlocked in Invasion would be how new vehicles would be made to spawn and respawn.
Spawning choices were added into the gameplay already, sticking with the model from the beta for Invasion and Invasion Slayer, or the refined model that will be present in the final build of the game, would seem pertinent.
To be honest, modifying an Invasion Gametype for use on Custom-made (HUGE) maps in Forge World (or the huge and empty/flat map, along the lines of the attic-ish area of sandbox minus the lack of floor, that I'm sure is in the mix or works), could make this Idea a reality. The only major thing that wouldn't happen would be the 20 v 20 games. I also understand that stealing a device after deactivating a few barriers isn't the same as getting massive numbers of kills and then blowing their base to kingdom come and that we might not even be able to create our own Invasion Maps due to the seemingly required placement of the "escape-phantom."
In lieu of being able to directly embed a Campaign Scoring Style points system or to be able to further make it applicable to single persons, one could then, in the gametype settings, apply bonuses for sprees, leader kills, assassinations, headshots, etc. This would, in effect, cause Tiers to be earned in a style that would easily reflect the truly wanted system. One thing that would have to change if this were to be implemented would be the number of points needed to win, or, at least, advance Tiers. Remember that the game does not advance via kills, but the points those kills earn (Kills in Reach can be set to be worth, both positive and negative, 1,2,5,10 and 0 points, so can betrayals, suicides and all bonuses). This being said, I would most likely advance the required points from 200-250 to 400-500 (the maximum).
I really do hope that Bungie decides to take this and develop it with their vast toolbox, as it far outshines anything we would be able to do with the tools they have given us, but if they do not, or until they do, I don't think we sholud have to live on air. I thank the original poster and those who provided ideas along the path for inspiration, and will be making note of them in the descriptions of the Custom Content.
I will be developing said gametype and, if possible, maps, starting the day that Reach is released in the early afternoon, Central Time (as if it matters). My GT is The Ardly374, same as my UN here. Message me here on Bungie.net if you would like to be involved or would like to receive notice when something is finished and I will do my best to make that happen.