There was something wonderful about the surprise monsters, which I hope we'd be able to isolate and replicate for future generations. When the SoF ambush revealed itself, and blasts of fire consumed any dwarf standing outside, it was extremely awesome, and possibly the most memorable thing about my entire turn. Yet, it was also terminal, so it wasn't canon or very useful. How can we keep something not unlike the surprise of swarming SoFs without making the game unplayable?
One thing I think we should try is a no shut-in rule. There must always be a path from the edge of the map to the heart of the fortress. This ensures that players have to defend themselves competently, instead of just shutting the doors and riding through impending starvation and madness. Combined with challenging enemies, this rule could increase fun for both the players and the readers.
Another thing is more modding, with an emphasis on diverse, surprising and challenging enemies. This could be accomplished by starting the game with many enemy races and agreeing not to peek, or by allowing players to mod the RAWs and then submit them along with the saves. I'm not experienced enough with modding to know how far you can mod a race after worldgen, but it's at least far enough to allow for different stats and abilities, if not a new name and symbol.