But when you use the words "shallow" and "depth", I have a feeling you mean there are no skills, no races, no mutations/religion, etc.
Not at all. Complexity and depth are different things. Go, for example, is a profoundly simple game that can be taught to a child in an hour, but it's got enough depth that one could study it their entire life and still have room for improvement. It's good that you can distinguish between the two.
Anyway, as far as improving GKH goes, one of the easiest ways to make bump combat interesting and tactical is to ask the player to deal with squads of enemies that stay together. It makes location more important (open spaces mean you get swarmed, but hallways can lead to becoming trapped on both sides), it brings in the concept of target prioritization, and makes hit and run tactics more viable.
The two biggest problems that stand out to me in GKH are its food clock and the way it handles player speed.
A proper time limit gives meaning to even the most basic decisions. Like in Brogue, the enemies are dangerous, so I wanna stop and heal between every fight, but the food clock is really strict and if I do that I will starve to death. The simple choice of how much time to spend resting between fights can really differentiate between strong and weak players.
Golden Krone Hotel has a really minor time limit that doesn't need to be taken into account very often. iirc vampires never starve so you're free to wait around as long as you like until an ideal situation presents itself. A real food clock of some kind could make a big difference. It doesn't even have to be about food. If powerful but unrewarding enemies begin to generate the longer you spend in one stage then you should do your best to move on quickly.
The speed system, especially the fact that dexterity increases your movement speed leads to a bunch of problems. Any enemy who is slower than you and who cannot harm you from range can be beaten 100% of the time in absolute safety as long as you're willing to put up with the tedium. On the other hand, if an enemy is faster than you, running away is suddenly not much of an option. You won't escape and you'll take damage without giving any in return. Do enemies in GKH follow you through stairs? I don't remember. If they do then it's not as bad. Anyway, it's one case where the game could be improved through becoming even more simple. Give human form a set speed and vampire form a set speed, and don't include any easy ways to alter them.