There are a plethora of things that could be added to make SK that bit more interesting.

Have monsters with multiple attack types. I'm not talking 2 or 3 like lichens or gremlins, but how about 6 or 7? Every monster having a ranged attack (be it weak or strong), a close range attack, a status inducing move, a healing move etc.

Imagine if a group of 5 constructs came towards you; if they had multiple attacks you'd have to think that bit more before charging in and the monsters themselves would be a little more interesting to fight. Instead of a hoard of enemies coming towards you attacking you have a TEAM of enemies, that work together.

Puzzle based bonus rooms in levels. No I don't mean shoot a random block and a key appears, or gimmicky ghost blocks leading to treasure you can't even see on screen, but how about boulder puzzles (cough pokemon cough) in which if you get it right, you can get a treat, but wrong and you'll simply have to wait till you come across that puzzle again or maybe actual maze puzzles with time limits to complete.

Making general gameplay more interesting, how about challenges at the start of depths. A vendor offers challenges with sets and weapons, which, should you accept, gives you the loadout they offer in return for increased loot and drop rates.

Weapon modifiers. With the lack of overall weapons, why not add modifiers that alter the way weapons work. For example, you can increase your attack speed on a weapon, but the damage or range decreases. You can increase the bullet speed of a gun but its reload time is longer. Talk about the endless possibilites this could create.

So these are just a few things I thought of off of the top of my head to make gameplay more interesting which will never get implemented, but your thoughts anyway?