During the latest Nintendo financial meeting the following picture was shown:

https://pbs.twimg.com/media/C3i4MteWEAAIv_N.jpg:large
Going by the order of the covers it looks like ARMS will come out between MK8 Deluxe and Splatoon 2, hence likely in late May to set apart the releases of the games evenly. It's to be noted that in this scenario ARMS would follow the path of the original Splatoon which released in late May as a new IP, with a lower pricetag and the promise of additional content to come later on.










