To increase difficulty all they need to do is put it back to what it was the day H&H dropped where you had hardly any stamina and everything cost too much stamina to use. That made the game very difficult... I think a lot of players, including myself, actually stopped playing for a time since they didn't find it fun to play anymore.

Of course other easy ways to increase difficulty would be...
Reduce armor ability to block damage
Reduce food effects
Reduce speed of player
Remove portals
Remove cooking upgrades
Remove parry
Remove Crits
Remove tombstone just lose all gear
Perma death lose world and character on death

I'm sure I can think of lots more, this is just off the top of my head.

Reduce skill gain
No map markers except player made ones
No mini map
No maps at all
No trader
Half inventory space
You have to pick up 100 of any item to get the recipes from it
Make the wind AI actually work against you
Any hit taken from Mountain creatures causes a slow effect on player that can stack.
When creatures die they put out a death cloud that makes drop recovery impossible for 10 real mins. So you have to wait to collect any drops.

Just to add I am not personally wanting any of this ... I am just trying to think of painful things for people who like pain might like. Most of these things would make me stop playing at all and think the dev went insane.

Well they could remove parry and crits and I probably wouldn't care....