A few things that I think that would be nice:
Add other kinds of potions to the alchemist. I've been playing for a couple of days and now my alchemist is at level 15. It took me some time, but I just noticed that the only things that he produces are healing potions. So, what I suggest is that he could make other kind of potions, like a "Attack potion", "Accuracy Potion", a "Quick Potion" (to make them end a quest faster) and so on. They could last for 1 quest or more. Also, like the "Cure Potion", they'd get better with the alchemist level. They could require a determinate amount of moss and brush + other items.
Add more selling/loot slot, scrolling them to the sides. I noticed that you said that you have some ideas that are limited because of the phone's screen size. Have you considered, for example, making the spots for selling scrollable to the sides? So you could add more selling spots. Same as the loot. That would allow getting more loot, other than only 4 per quest.
Display equipment’s' information in the inventory with a window. I read in here that you thought about giving bonuses if a character is using equipment’s from the same categories, but didn’t know how to display that. You could add a window, like these that appears when you click on your character's equipment, or in a equipment in the recipe's window. So, in the inventory, when clicking equipment, it could display a window with its information.
Exploring quest. There could be an exploring area, where your hero could stay like na hour and fight random monsters from the area you are, giving you loot from these random monster, and, who knows, equipment, accessories, etc. But I guess that this would only be possible with more loot slots.
Limit character's equipment based on their class.
If using an equipment set, characters visual could change to that set.
A sorting system for the items.
To make it a little bit more challenging, there could be recipes that would require equipment as an ingredient, like to craft a rare item.