Lich is still babymode. It's kind of weird with him, they've shown that Lich is still totally still inside, but not a lot of urgency is present with that and I guess Lich is either powerless to get out or waiting for something. It seems likely he'll have one more showing before the end.
There's been a few smaller villains since then, like an ice elemental from before the war, some human scientist who likes making robot/person cyborgs and shit, and now PB's uncle is back doing stuff.
Hmm, I don't know what would be considered twists, there's definitely been some but nothing I can remember. Well I guess the fact that Ice King's penguin was actually an ancient alien warlord counts but that was just stupid so it's not like it's a good twist.
And yeah, more humans are out there. Finn went and found a series of human islands, most of which fell into disrepair but there's one ruled by his mom (or an artificial construct based on her, depending your view on it) and they had some bonding, it was cute.