House ruling is fine but shouldn't that level of challenge (either through an enforced time limit or some other means) exist in the vanilla game?
It may be, in effect, a customisable game but that feels accidental rather than anything the designer intended. I could make any game harder by drinking heavily before playing it but that external factor making the game more difficult hasn't really improved it as a game.
Worth saying, I'm not wild about either game. For me they're simply not rewarding and I found myself resenting rather than enjoying the time I spent on them.
I just tried it out again and I used a slightly modified version of the technique laid out in this article (Left-Down-Left-Down until I couldn't anymore then switched to Right-Down, rinse repeat only occasionally switching directions to setup long number chains ) and I got a 4096.
For you maybe, not me - not all the time anyway. Granted when I am bored and feeling lazy, or I have a quick subway ride, I'll spin up 2048, but if I want a challenge and some thinking, I'll spin up Threes. In some sense, 2048 for me now serves as a less twitchy, less exciting Tetris substitute.
Moving the goal posts back on behalf of a game makes the goal more difficult, sure, but it doesn't necessarily mean the difficulty of the actual gameplay increases at the same rate, i.e. it can lag behind the difficulty of the goal, which in my opinion is very much the case for 2048.
And as I mentioned in my original post, the goal for me to get 4096 or over 50,000 points. 2048 is easy, 50,000 points isn't.