I've completed all the rank missions, heated my 5* gear to level 10, got my sprite to level 100, done Operation Crimson Hammer, done a shadow lair, and have all my desired gear. Now what do I do? I still find the game fun, but I can't find anything to do. In any other game I'd just start a new save, but getting to the end in SK is such an investment that is just isn't worth it. The endgame feels way too lacking and anticlimactic for how much work you have to put in to get to it. I've just been doing FSC like once or twice a day while I rack up crowns that I don't know how to spend.
Funnily enough, a popular thread by Reydann was posted beneath yours earlier discussing this very problem.
Luckily enough, a dev replied and confirmed that they'll be working on incentivising players to engage with all of the game's avenues after they modernize the game (their current focus).
I recommend gathering friends or guildmates and running a super chill Arcade session for the hell of it. No speedrunning, no loot focus, just vibes and an active background to a chat or hangout.
Not much of a solution or a thing to do until then but can be enjoyable if you're with the right people and you'll still be getting a smattering of loot regardless. Playing something else while keeping a close eye on the game until it updates also can't hurt, whatever will pass the time for you.
Until then!