Well, if it's farmville-esq, then I imagine it'll be online - so it'll be pretty much automatically a cross-platform game. If that isn't in the cards, I'd like to suggest it up front.
Otherwise, surely you've thought of this sort of thing:
1. Ability to trade w/other players
2. Ability to "buy/sell" at store
3. If it's a "pay for" game, perhaps you can build in a free and paid version? Not sure this'd be a "feature" just an idea.
4. Zoomable, rotatable, movable - in other words easy to navigate - map
5. Built-in Chat, which I suppose is pretty much a given with an online game in our day.
6. Language filters - if/where useful. For instance, if there is any foul language in any script, some people would want to be able to filter the foul language.
7. Ways to "friend" other players and block other players from chat, etc.
8. Parental controls/accounts. With younger and younger players playing all the time, this would be good. I think of on the Wii, you can set up parental controls to allow/disallow Internet connection, when can connect, whether the child can buy stuff or not, or how much they can spend... etc...
9. Rating system if applicable - though this would more likely be a per game rather than per game portion deal.
10. Silly one-liner jokes/sayings IF/when they fit - say for farm animals, people/characters...
11. Instead of robotic help language, make any language more human, possibly humorous when fitting.
Well, give other ideas or suggestions, I'm sure, but I'd better cut off the idea tap before this response gets too long.