If there would be one single thing I wish could be added to Seasons, it would be that the Sentiments from the festivals in Snowy Escape would also be added to the main calendar holidays that automatically are added to the calendar with the pack. I understand it would be difficult, maybe too difficult, to add customizable Sentiments for custom holidays. But if we could at least get Sentiments for the main holidays, that would make playing them a lot more rewarding.
If there would be a second thing I wish could be added, it would be to second what Speedy1236 said and that there would be a way to use the Pride objects with a holiday more easily. Or more generally, tying objects to holidays. Right now, it's rather difficult to create custom holidays that use objects to their fullest, and that could be improved.