My suggestions
1: shopkeepers body progress being linked to their inventory an example is the bunny shopkeepers inventory expands every time her artwork changes allowing more items to be bought and sold (though you have to buy the new stuff to expand again just to prevent people from buying a shit ton of fishing bait)
2: new skills being alchemy and cooking (and others I've yet to think of)
alchemy give the player the ability to make buff and debuff potions using the fish or nightshade they will acquire
cooking makes stronger healing items as well as granting permanent exp and money stat increases (by use of hard to acquire ingredients)
each having their own npc
3: either a revamped fishing minigame (due to the sheer amount of hand pains I experience doing the old minigame) like a clicker of a timing minigame or an item bought by maris that (with a % chance of working based on the fishing level meaning you still need to do the) auto fishes for you base on how much bait you used (30 bait = 30 times with a chance of nothing be fished)
4: more sellable goods like the charms like stone into sculptures and bronze (and other metals) into goods