Here's what I'd like: in the editor, you can add types of tiles (assuming you want to make the graphics). Firaxis includes what they have now, plus some newer, nicer graphics. You choose what a tile's actions can be, and what that tile turns into when a particular action is used. You also choose tech required, # of turns, etc.
In addition, you could choose the pictures on a per-era basis. So if you don't want new worker actions, you just use the farm graphic for middle ages and on...
This way, grassland could be irrigated to "grassland-irr" or something. If you want, you put in refrigeration and make "grassland-irr" able to be irrigated to "grassland-farm" or something.
And that would make other mods, with things like terraforming, doable: say you don't want tundra. Well, you research terraforming in the modern age, and you add "terraform" to the tundra's worker actions. The action takes 40 turns and produces a plains tile.
I would LOVE to see something like that added. Anybody else think that would be the best solution?
EDIT: Also, the AI would evaluate the actions not based on "irrigation = good for cities needing food, mining = good for cities needing production", but based on something more dynamic. IE, the AI evaluates workers actions based on the end result. So if you put in an action called "irrimine" that can be used only on hills and turns hills into "mined, farmed hills" (+1 food and +1 shield), the AI evaluates the usefulness of these "mined, farmed hills". The AI takes into consideration all potential actions, time necessary for actions, what the city currently needs (or perhaps what it feels would be best in the long run), and bases its decision on that.
And in a perfect world, the AI would also look at what these "mined, farmed hills" could turn into. So if you had another action called "automate farming" available only on "mined, farmed hills" (+1 food again), the AI would take that into consideration before "irrimining" its hills.
Okay, so this is very complex and never likely to be added, but still, it would be nice...