My idea of the BIGGEST thing DF needs is a tighter tying together of the three "game modes" as I see them:
Fortress
Adventure
Worldgen.
"How is worldgen a game type?" you say? Easilly, it controls the entire world in an abstracted way. It has it's rules, it's own displays, etc. It's not very interactive, but it's a game mode.
Now, consider this. What would happen if, when in Adventure mode, Worldgen continued running at a much slowed rate. People moved from place to place, megabeasts attacked, wars were fought. It wouldn't take too much processing power to have the world right outside of where the player exists keep being generated. It takes maybe 30 seconds per game year to calculate the events for a 256x256 large world with a population exceeding 3 million people, on my computer. To calculate the events of a single day would take almost no time at all, and when you consider that game day usually takes about 5 minutes to play in game? 30 seconds / 365 days / 5 minutes = negligible calculation time.
And the world would be so much more lively! It would lead to the future possibilities of being able to SEE the wars (which currently you could not really see), watch sites being placed. Right now you could probably only leave a place, then come back after a year to see it has changed... which would actually be freaking amazing. Just keep the same seed and roll the dice again.
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Second tie in between the modes: Fortress Adventurers. Put your site on hold to take an adventurer out. A random able bodied Dwarf from your fortress would be selected and set out to pasture. Your fortress would basically be paused besides some basic automations, you could buy/sell/steal whatever you need for your adventure from what's already at your Fortress (that your Adventurer spawns at), and could set out! Or not! Go down into the caverns from your 50z high staircase... Recruit your whole army as followers and set out to take out those megabeasts that KEEP coming from that cave you embarked 3 tiles away from. Etc etc. Then, when your adventurer dies or you retire him, you can start the fortress back up again.
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Third tie in, but this is only really about one game mode. Drop out of worldgen and change settings, then start it back up again. Change the site cap, population cap. Place more civilizations. Place more megabeasts. Basically put the player in control of the world generation process and make it into a TRUE game mode. Declare, as God Almighty, and a site be placed... here. Make it a tower. Etc etc. Think about it for a good solid minute and think about how f-n awesome that would be.