First idea.
Please do this, allow players to post the maps they made. Put the maps available for review for a month somewhere on the game. Let the players vote on the maps. Allow players to score them 1 to 100. If it makes it 85 and above it gets put into the game, it costs 343 nothing the player gets full exposure for making it and a possible notch on his resume. Players play maps they want, Everybody wins. This should be done for all gametypes. The motorcycle game Trials has some really awesome maps that have been created by the players.
This alone could make Halo 5 one of the, if not the best Halo ever.
Second idea.
It would be really cool would be if they would take the best H2 Anniversary small team maps and supersize them for multiple vehicles to pass thru most areas at the same time. They would be a blast to play. This could be done maybe by using Forge and done by some players.