There's pretty much a dead cert Sheldon's Picks will be mentioned and teased, possibly unveiling some kits with a release date for next patch. I just wonder if we'll get the glorious Wasabi incident from Splatoon 1, where NA and JP showed the kit as Splat Bomb/Inkstrike whilst Nintendo of Europe decided to tell us the Wasabi Splattershot came with Splat Bomb/Inkzooka instead :P
Outside of that I'm not expecting much for Splatoon 2, since major content is all wrapped up now and we just have 5 months of Splatfests left (last one is going to be July, what with the 2 years of Splatfest promise). We'll probably see a few of the "unannounced and almost ready" titles pop up in this Direct tomorrow. Not sure what Fire Emblem: Three Houses (news to me this game even was announced or mentioned), but if that's getting a 10 min slot or whatever we probably will get a few announcements and nothing much else. Will still be nice to see 2019's release year get populated, because a lot of games just have a 2019 release (no longer including Metroid) with nothing more concrete in store.
As usual with Nintendo and announcements, try to keep your expectations in line, no point in overhyping unbelievable rumours only to be let down and call a decent direct bad because of it.