Here, have some funny displacer beast-related content.
Quite surprisingly, displacer beasts already have a LA. Normal ones are at +4: pack lords are apparently too strong to allow as PC's (oh WotC, will you ever learn?). It should be obvious that this is too much.
First things first: displacer beast have decent stats, ranging from 18 (strength) to 5 (intelligence). Large size helps, though only their tentacles have actual reach. Apart from the two aforementioned tentacles, they also get a bite attack best used on a mouthpick weapon.
The special abilities are less bland: resistance to ranged attacks is a small situational bonus (albeit a pretty good one). Displacement is pretty awesome: it gives about everything a 50% chance to miss. Sure, there's a few ways of blocking it, but those are lacked by many monsters.
However, in the end I don't think DB's deserve a LA. Sorcerers, wizards, beguilers and even some clerics and druids can already cast Displacement several times a day, and as a purely passive defensive ability I can't help but wonder how useful it'll actually be.
I remain open to suggestions, and I could see people wanting to give a +1 LA here. If so, do not hesitate to tell.
Displacer Beast Pack Lord
Oh no! It's a displacer beast with a size increase and some ability boosts! What havoc will it wreck amongst the CR 18 monsters! -0 LA.