Okay, a couple of points:
- Uhm, I don't know if some of you are familiar with the concept of seasons? Because when it's summer where I live, it sure as heck isn't winter 20 miles down the road. How could you possibly have route-only seasons?? I mean, I can understand caves being immune to season changes, or mountain-tops being snowy most of the year, but you can't walk from an autumn route into a spring route. That'd be madness. If they are introducing seasons, it'll have to work all over Isshu, like the day and night feature.
- Seasons don't have to be in real time, just like how playing a soccer/football computer game isn't literally 90 minutes long. Seasons could change every week, or every two weeks. That way, you'll play through the seasons every month and have lots of variety.
- Seasonal evolutions would be great! Why not? It's an interesting method of evolution. It would've made heaps more sense for Eevee to evolve into Glaceon in Winter or evolve into Leafeon in Summer, instead of battling near a rock.
- There could be loads of cool changes. More bugs in Spring and Summer, more Ice-types in Winter, hibernating animals unobtainable in Winter, migratory birds like Taillow or that new swan/goose/stork pokemon available in Summer/Autumn only, less pokemon battles in forests during Winter and more in other months, etc.
- Maybe certain areas would only be available in a certain season. Perhaps a mountain area is unpassable in the heavy snow depths of Winter, or caves only open up when water levels fall in Summer, or an area becomes Surf-able after heavy rain in Spring - the possibilities are endless!