Grandmaster Nightfalls will be 1050 Power activities, which is 10 levels lower than what Bungie originally suggested. You'll need to be 1025 Power to even attempt them, and because they use a similar contest modifier to the one created for world-first raid races, your Power will always be capped at 1025. In other words, you'll be at least 25 Power underleveled no matter what. This will keep the playing field even (and the challenge steep), and it means players won't feel pressured to grind bounties to over-level their Artifact to offset the Power requirement.
A few new and returning modifiers will also be introduced with Grandmaster difficulty. Extinguish, which returns you to orbit if your team wipes, is back after being relegated to the old Nightfall card system. It's also been paired with Limited Revives, which - as you may have deduced - starts you with a limited number of revives which you can increase by killing Champions. Speaking of Champions: Bungie hasn't confirmed it, but data miners believe Grandmaster mode will also have a modifier that increases the number of Champions in each Strike.
Bungie wants Grandmaster Nightfalls to push build crafting and coordination more than any activity in the game, and it believes that "completion should be the primary metric for success, rather than score or time or another in-activity metric like ‘killing all Champions’". Curiously, it also stressed that "Grandmaster should be watchable," presumably alluding to Twitch and YouTube content. However, it says that "the main draw of the mode is the new Seal and title available for beating each of the Grandmaster Ordeal Strikes once during the Season: Conqueror."
As for more tangible rewards, Grandmaster difficulty will grant an "increased chance of being awarded Exotic armor and Masterworking materials" like Enhancement Prisms and Ascendant Shards. It remains to be seen whether an increased chance at materials which are already available from easier activities will be enough to keep players grinding Grandmasters.