- Halo 2 Rookie
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- Exalted Mythic Member
I think the biggest thing to help replayability is if the fights in the campaign story (or stories, you never know) are not predetermined; perhaps, almost like in Left 4 Dead, it's a matter of the game following a broad guideline on chance and what is fair.
Starting a level and finding that this time there's a couple more Hunters in that group of enemies would be nice. As long as there's also a chance that anything else could happen.
Heck, if this is an honest wish list, I think having the same Left 4 Dead 2 directive scheme -- where the game adjudicates how the level is laid out based on the quality of your fights -- would work splendidly to make campaign interesting again and again.
Leaderboards are a good way to inspire replayability. Leaderboards for firefight maps and campaign metagame levels. I know the leaderboards alone encourage me to still jump on Batman: Arkham Asylum now and again.