Once you have done the main questline in Suramar, you can still gain rep with the Nightfallen in a lot of different ways, such as the ones below:
1) Complete the event "Whithered Army training". This scenario can be completed for free one time, and the other times you must spend Ancient Mana to do it. You should run it at 2 000 Ancient Mana for the best amount of rep gain. You can read a guide on how to farm Ancient Mana very easy here! This also gives you a good amount of Artifact Power, so you should do this as much as possible once you hit 110.
2) Complete World Quests that are tied to Suramar and the Nightfallen. You can mouse-over the world quest to see which faction they are tied to, and any quest tied to the Nightfallen will award rep with them.
3) The endbosses of the dungeons Court of Stars and The Arcway awards +250 rep for each kill. This can be farmed, but takes a long time to get a good amount of rep.
4) Do Kirin Tor world quests that are tied with Suramar. This gives you rep with both Kirin Tor and The Nightfallen at the same time.
5) Do The Warden World quests. Just like with Kirin Tor, these quests gives you rep with both the Wardens and the Nightfallen.
6) Do Kirin Tor Emissary Quests. After completing four of these, you get to pick a faction to increase your reputation with, and you can choose to get +750 Nightfallen rep!
The Nightfallen reputation rewardsThe Quartermaster sells stuff once your reach certain reputation levels. She is called The Arcanist Thalyssra, and is found in Shal'Aran in Suramar.
Cloth - Feet
Leather - Feet
Footpads of the Nightrunners
Mail - Feet
Scavenged Felsoul Sabatons
Plate - Feet
Treasure Map - Suramar
Reveals treasures on the minimap of Suramar as long as the map is in the inventory.
Mobile Telemancy Beacon
Set up the beacon in the field to return to Shal´aran.
Boon of the Manaseeker
Permanently enchants shoulders with the Manaseeker enchantment, allowing the wearer to obtain ManaTinged Packs from corpses of their enemies.
Braided Manastring Cinch
Cloth - Waist
Requires lvl 110
Class Hall tier - Hands
Home Made Party
Don a “Home-made” Nightborne party mask
Stam, crit, mastery
Class Hall tier - Shoulder
Pattern: Imbued Silkweave Bracers Rank 3
Pattern: Imbued Silkweave Pantaloons Rank 3
Pattern: Imbued Silkweave Slippers Rank 3
Design: Subtle Shadowruby Pendant Rank 4
Formula: Enchant Ring – Binding of Versatility Rank 3
Formula: Enchant Ring – Binding of Mastery Rank 3
Formula: Enchant Ring – Binding of Critical Strike Rank 3
Formula: Enchant Cloak – Binding of Intellect Rank 3
Class Hall Armor Tier