Well, let's look at the difference between Wrath and Cata.
Not including Legendaries, and correlating ToC to FL and ICC to DW raid.
2nd to last raid tier of Wrath: Highest item level available: 272
Last raid tier of Wrath: Highest item level available: 284
So, assuming we'll see the same 12 point jump between Rag and Deathwing (OR the boss that comes after in a Halion-esque pre-expansion raid):
2nd to last raid tier of Cata: Highest item level available: 397
Last raid tier of Cata: Highest item level available: 409
Between end tier Wrath gear and entry tier Cata raid gear, there was a 75-point difference (284 --> 359)
So, we might be able to assume that the item level for entry tier raiding gear in the next expansion will be 484. IF the next expansion has the exact same number of tiers as Cata, AND the same number of levels (5), then it's not too unreasonable to assume that raid gear will start at 484 and end at 534. So, at some point next expansion, probably after the introduction of a new raid tier, they will implement a new BS pattern. I don't necessarily think that it will be strictly necessary to implement a new one every expansion.
In addition, considering that 484 is the smallest possible estimate of the starting ilevel of next-expansion raid gear, it isn't actually a hugh item level jump if we see it pushed to 500+. It's there, but it's not game breaking.