I guess for me it was Mario Kart 8's DLC content, but like others have said, the Switch port negates it by having them in the base game AND adding even more characters and an improved Battle Mode. And didn't they say we were getting more DLC for Mario Kart 8 Deluxe? All we've gotten so far that really counts is Link in the Champion's Tunic with the Master Cycle Zero. Great additions, yes, but that was months ago!
My other pick would be the large influx of playable characters we've been getting for Mario Tennis Aces!