Also: fishing.
No, not whale fishing, I mean the outdoor activity. In fact, I'm almost shocked they haven't gone there yet. It's probably the most classic in-game minigame and can be implemented from dumb (click, wait, click again when it's bobbing, get RNG fish) to high end (choice of hook, weights, lure, fish behaviour modeling, mouse gestures for line handling, etc) with everything in-between, and gives people something to do when there's nothing to do. I should know, I love in-game fishing as silly as it might be, from the dumb LotRO implementation to the fancy ones like Ice Lakes or the aiming-based Warframe spear fishing. And SC being SC you'd imagine they'd implement (or, well, promise at least) float fishing, lure fishing, spear fishing, bow fishing, underwater speargun fishing, etc. Heck, they could sell fish trawler JPGs and have a box of fish delivery mission tied to it. As for the commercial/marketing side, well, just have a look at the available dedicated fishing games on pc: 99% are online multiplayer games riddled with microtransactions, so well, in addition to mmo fishing being a proven time-sink, there seems to be a market for it too.