You get nothing for beating the game in general.
Which honestly annoys me a lot.
Halo 3 Legendary gave us EOD Armor
Halo 3 ODST Legendary gave us Veronica Dare for Firefight
Halo 4 Legendary gave us Mark VI
But Halo 5 and Halo Infinite on Legendary gives us nothing. Players wanted to be able to wear the Mark VI MOD that Master Chief wears in the campaign of Halo 5, but we didn’t get it. And players REALLY want to play with the Mark VI Gen-3 Armor Core, but we get nothing for Halo Infinite’s campaign completion.
Perhaps we will actually get something when the ENTIRE campaign is available for completion. So far we are only playing Part 1 of Halo Infinite. Who knows what unlocks we will get in Part 2 and Part 3, or even any of the potential mini-DLC Campaigns that we should be getting sometime down the line.