I know that ice fishing has been mentioned by others in the past. In the northern US where I live, ice fishing is very popular. I would pay for an ice fishing DLC. You could bounce between different fish houses on the lakes just like the different pegs on some of the existing summer lakes. Most of the fish species are already in game. Could add some trout. It could even have a flasher style locator too. It's just an idea.
Would be a nice change up from current lakes offered and would be cool once they got the premission needed to use the license gear needed for this if it was implemented as it would open up possibilities for more ice fishing venues from around world to be added