|Sitting on a stat spread of 39/46/30/37/18, Corrin's stats seem fairly unimpressive. However, one glance at that Defense stat shows where New Year's Corrin shines: He functions as a tanky Bow unit, physically, for those who don't have a Faye, Leon, or a Gordin they want to use. Being a free unit, Corrin has Neutral IVs permanently, which is a blessing and a curse. His stats always will turn out fairly balanced like this, but he excels in no regions. Focusing on his defense lets him abuse as much of his good traits as possible.
His Bow could be any number of things, but the Guard Bow gives him the best odds against units such as Reinhardt and Brave Lyn if packing Distant Defense 3 as his Seal slot. Slaying Bow+ gives him faster Bonfires and Ignises, while his standard Hama Ya+ isn't a terrible weapon with a ranged Spur effect. The Guard Bow, however, provides himself with a defense buff and, by technicality, a damage buff to his Bonfire/Ignis based on if the target is ranged.
The Assist is his standard Rally Def/Res, giving support to Blade Tomes while also providing a nice amount of Arena Points. If Rallies aren't your forte, a standard Draw Back or Reposition in this slot works.
His Special slot is Bonfire or Ignis, based on if damage or reliable proccing is your style of play. Bonfire does a hefty 18 damage, higher if buffed from Spurs or Hones, and only has a 2 charge if running a Slaying Bow. Ignis works with Quick Riptose due to how frequently you will be getting attacked and be counterattacking.
The A-slot is Close Counter, to make the most out of his strong Defense, similar to that of Beruka. Instead of initiating with Corrin, you should be waiting for the enemy to walk up them blow themselves up on him whenever possible. Rallies for Defense and Speed can help prevent Corrin from taking too much damage on a counterattack.
The B-Slot is either Quick Riptose or Vantage. If you like your Special activiating more often and doubling opponents, Quick Riptose is your best friend. If you like attacking first, even on enemy phase, Vantage is an option for you.
The C-Slot is flexible. Hones and Spurs are obvious choices, and Corrin himself comes with a strong Spur in Def/Res, bolstering the effect his stock Hama Ya has. However, the Tactic skills and Smokes also are effective if you're willing to sacrifice valuable units like Siegbert, Brave Lyn and Sigurd.
The Seal is also preference, although the Defense seals to me are incredibly strong options. Close Defense gives Corrin an effective 43 Defense before Spurs or Hones, making him better against units like Ayra, Mia, Brave Ike, and Effie (although don't expect to happily tank the latter of them all.) Distant Defense gives him better odds against Brave Lyn and non-Death Blow Reinhardts, however rare those are, as well as other Daggers and Bows like Bridal Cordelia and Sothe. Quick Riptose seal is also possible, combining the effects of Vantage when at low HP and Quick Riptose when at high Health to launch off one final hit before dying to an enemy's attack.
tl;dr This NY!Corrin build is intended to make him fairly tanky and primarily an enemy phase unit, able to take on close range and distant range opponents that attack physically fairly well while also being a strong team supporter through Rallies and the Spurs of a C-slot and his main weapon.