Congrats, nice choice with your new LE6920MPS with new Magpul SL furniture.
The new MPS MOE series models was not even shown in the Colt 2015 catalog but they were just added at the Colt site awhile back.
Since you already got some mags, ammo, maybe get a sling first there are plenty to choose from, popular brands are Magpul MS, Vickers VCAS, and Viking Tactics VTAC (or their LaRue Tactical padded version), two point slings.
The current trend in slings are the 2 point convertible to 1 point, Magpul and Vickers (made by Blue Force Gear) have these two to one point models and the Viking Tactics & LaRue version you could get a 2 to 1 conversion attachment.
Then you could add a Magpul M-Lok sling mount either QD sling swivel or the Magpul ParaClip type (which ever comes with your sling), next would be a Magpul M-Lok light mount for a Surefire light.
And when you're pretty good with your iron sights then it's time to get your Aimpoint red dot sight.