As it is now, tombs are death zones for even seasoned adventures. With little to no reward (what with the free loot in keeps and the like), why would anyone risk a trip into one of these except out of a death-wish? The challenge level is largely fine as it is. The risk and reward ratio needs to be balanced somehow, and these are my suggestions.
Vampire and undead players can currently just stroll through all tombs uninhibited, and reap all of the potential rewards I may list. As such, I think as an additional feature, some tombs could be inhabited by semi-sapient life and maybe even large groups of bandits, to mix things up a bit. Maybe even add rare abandoned tombs that can be found in wildernesses, to be inhabited by wildlife. In terrifying biomes, this could lead to interesting results.
Add gen'd artifact-esque item(s) to these tombs, along with the occasional masterwork item. One artifact per tomb, or one every two tombs, at least. (I would say even less artifacts, but keep in mind you could be finding artifact pig tail fiber socks.)
Additionally, most tombs, depending on what inhabits it (if anything), will have a lord or overseer, to some effect. A mummy would usually fill this role, otherwise, it would be the bandit leader or troglodyte lord. They would likely be carrying the artifact/masterwork item(s) that would otherwise be found in the tomb.
These would add unique challenges for weak and strong adventurers alike. It allows veteran and new players to literally treasure plunder in hopes of finding something extraordinary. This also would allow us to see new uses to stealth, wrestling and all semi-related stats. You could sneak through the trap-ridden tomb and avoid/silently incapacitate enemies while searching for these treasured items that may be among the usual piles of valuables. Perhaps you could grab that artifact platinum helmet off of the bandit leader's head as he sleeps, or plucking the elusive masterwork pig tail fiber sock off of the front step.