Splatoon is one of those games that from the side-lines it looks fun, but you think it'll get boring. But, once you get the remote in hand, you won't stop playing until you absolutely have to.
The best thing is how fast paced & equal it is. Even when you play your first online match & your level 1, you can still do really good because the game is more skill then gear. And you level up super fast, making it very fast pace (as I already said) and being able to play with every weapon pretty quickly (that makes it more fun ;3). This also means there's barely any grinding
The maps are also changing all the time, and each map has its own strategies & stuff. Learning all the maps & playing them is fun. The map change also makes it less repetitive (because playing the same map over & over=boring.)
Solo is super fun too. It's a pretty quick play-through, but it's really puzzly & fun, super enjoyable (of course online is way better.)
My favorite part is the idea & fun of it. I'm not getting bored of it. Even at level 31 with (basically) every weapon, I still just love playing even when there's (barely) anything to unlock. One of my biggest worries when I was gonna buy it that once I hit level 20 I'd get bored, but I started liking Splatoon even more once I hit level 20
I hope what I said helped! Also, I want XenoBlade X so bad, too