The only thing I would change is sprint and map design (to a certain degree), even tough sprint is not as intrusive as it was before, I will change sprint button and just have a max Spartan speed, meaning that the more you leave the movement button press the faster the Spartan will walk until reaching a specific velocity, but a speed slower than current sprint.
Yep I would keep everything very familiar with small sandbox changes (automatics small nerf, magnetism nerf, maybe make thruster more offensive, SC slight nerf etc) and just focus on content (more maps, better design, more environments, pve mode, flesh out warzone, fuckton of reqs, theatre improvements, forge improvements, campaign replayability, actual btb maps, player submitted skins etcccc).