Okay, I'm at lunch, so I'm kind of skimming here:

First, why would the Jedi hide a treasure/relic? After all, the best
protectors of powerful relics would be the current living Jedi
Masters, not some forgotten tombs. Jedis are not known to bury their
own history on purpose.

You know - and this is still just a work in progress - the person who hid the treasure might be the Infamous Jedi Pirate King NAME-GOES-HERE. Big, blustery - think Ash from Evil Dead but as a pirate. Was he a Jedi? Was he a Pirate? Was he a king? Who knows! Perhaps he did have a bit of the force (so I could use the awesome hourglass door idea) - but I think I'm leaning away from straight out Lost Jedi Bling. We've had a lot of that recently in the game.

You need strong underlyings to your plot device if you want it to be
anything more than "a dungeon for dungeon's sake".

Why a dungeon? I'm thinking that Infamous Jedi Pirate King was running around gathering up treasure, found these ruins way the hell out of the way on this planet nobody ever came to, stashed his bling and then went off to fight a mighty battle (where he got his ass kicked once and for all). He was fully intending to come back and pick it up, but it was lost after his death.

Okay. In either case, you need some protection. We established that it cannot be a random beast if it has been a long term guardian. Nor can it be a living creature, period. It would have needed to feed itself or procreate..

I'm kind of leaning towards hydrospanner's thinking - yeah a big monster at the back end of the dungeon is a bit improbable, but Star Wars is full of goofy and improbable stuff anyway, so one more - a classic trope at that - probably wont break suspension of disbelief. As for sustainability of the ecosystem, perhaps he has a very nice garden just outside the back door that he tends when not fighting off adventurers?

(Why don't the PCs come in the back door then? Its obvious isn't it - they didn't know about it!)

I could easily see a large set of ruins where said pirate king either found a "beastie" already in residence and tamed him, or tamed said foul beastie elsewhere and brought him to his fancy new ruin digs specifically to serve as guard beast.

When the issue of care & feeding came up, suddenly the ruin's problem of being overrun by some other, smaller nasties changed from an issue to a solution.

Instead of goblins, cave trolls, and a balrog in Moria, I'm thinking some combination of womprats, reek, and a krayt dragon. The womp rats are no big deal individually, but can be overwhelming in large numbers. The reek will probably be encountered alone or in a small herd-family, but are a major concern, and the krayt is running the show.

I could easily see a large set of ruins where said pirate king either found a "beastie" already in residence and tamed him, or tamed said foul beastie elsewhere and brought him to his fancy new ruin digs specifically to serve as guard beast.

Oooh, perhaps the Pirate King saved the Monster's life in some manner or another, and he swore some kind of life debt sorta thing, why it hangs around and guards the treasure - he gave his word! That plays into the hideous exterior, brilliant interior I was looking for.

Depending on just how fast & loose you wish to interpret the force, it's also possible for the beastie to literally be your pirate king.

Perhaps they were in some sort of peril together, the beastie is on it's last legs and your Jedi pirate king isn't much better. In the instant of death, the Jedi completes some sort of melding ritual which binds his force spirit to the beastie. Perhaps the beastie WAS a savage brute until this happened, at which point, it gained the Jedi's consciousness?

I guess a Jedi Pirate King (Jedi in a somewhat loose sense) would explain a lot of the reasons why these goodies would be buried.

Hell, nothing to say that the Tomb wasnt built by a Dark Jedi who thought he was still a Jedi.. Theres a lot of iffy moments in Jedi history. Wasnt there a royal bloodline somewhere who ended up being Dark Force Users?

So i can see where you are coming from; and its fine. I just latched a bit strong on the idea that this was a JEDI temple, and assumed it had to be Vanilla Jedis as we know them. Plus, i am a sucker for verssimilitude.