Ability Scores. These are already vestigial. The only thing that matters is the bonus. Unfortunately.
Classes. These are likely to last a good long while. They're archetypes and as such have more "pull" with the fans and potential fans than some of the others. Likely one of the last cows standing.
Races. With the recent changes and options, race is pushing closer and closer to meaninglessness. But they're also archetypes, so they're in the running for last cow standing.
Hit Points. With the high amount of healing, low monster damage, and encounter design so wildly in favor of the PCs, hp are basically pointless book keeping after the first 2-3 levels.
Vancian Magic. With the proliferation of non-vancian casters WotC has set up the possibility to de-emphasize vancian casting. I think it will last a long, long while before it finally goes. It's too iconically D&D.
XP. It's basically already on the way out with milestones and story-based advancement.
Levels. XP is only relevant in regards to levels, so there's no point in tracking the same info twice. Like ability scores. The only thing that matters is the modifier, not the score...the only thing that matters is the level, not the XP. So levels will last longer than XP, but will levels outlast the other stuff?
The DM. There are 2-3 things that define something as an RPG. One is having a Referee, GM, or DM to run the world. D&D already tried a soft elimination of the DM in 4E and that didn't seem to go over very well. So this will likely be the last cow standing.