Luke's facial expression is a reaction shot to the fully revealed creature, it's a really nice piece of non verbal acting on Mark's part. He is channeling Sir Alec. You can almost hear him thinking "mmm...that's a bit nasty, oh well all the days work of a Jedi...maybe I should have dropped Ben off at Anchorhead" anyone else would have gone OTT, it almost sells the current footage. To get the maximum effect from Luke's reaction the creature need to be a bit more of a monster so when we look down we gasp at how hideous it is and Luke does his "here we go again" look.

A lot could be done with just sound, the rumblings of some vast creature bursting up from the depths.

When the skiff gets hit it could knock it closer to the Sarlacc so it can pull people off the deck with it's tentacles Jules Verne style.

It would also be fun to have the bad guys swing over on ropes (especially if King Of The Rocket Men is busy elsewhere) it would add to that swashbuckling feel and set up what Luke and Leia do later.

If the sail barge destruction was more like Dave's video mockup. the creature could pull people out of the Sail Barge too as it collapses, and then duck back into the hole to survive the explosion.

Slightly changing the subject, the Weequay that Luke defenestrates sounds too human and it's almost like he has stubbed his toe rather than he is plunging to he death.

There were plenty of alien falling noises made for Jedi Outcast which would work better.