This is a great idea! Definitely works well with the Zelda aesthetic + reference as well. Going to consider it!
It's good I checked through these posts first because I was going to suggest something very similar. At least some way of stacking coins or large sums of items into a single item. I did have fun throwing all of my stone into the market at once through
I'm sure you are aware of this bug, but maybe not this variation of it. You can place a building half-way between tiles 
After breaking these buildings, you can walk over the water or whatever occupied that tile.
Also, at least 3 of the upgrades (Tactics, Resolve and Alacrity) require you to unlock at least one upgrade (Shielding) that is effectively useless.
For the purposes of the demo, is there some way that upgrades that aren't implemented could be bought for free?