Bert, it seems to me, is mostly a character for characters we care about more, like Al, like the Doc and Louisa, to play off...which makes me wonder if S6 may be the time to make him central to some multi-episode subplot (like his battle with a chronic illness) and then to kill him off.
I remember when J.K. Rowling killed off Albus Dumbledore at the end of book 6 I thought, how can there possibly be a wizarding world without him? Reaching back further into my childhood reading, I remember when L.M. Montgomery killed off Matthew toward the end of Anne of Green Gables, I thought, how can "the Anne books" (of which there are at least 5 subsequent) have gone on without him? The way I justified both those deaths to myself as a reader (and I am still happier about Matthew's than about Dumbledore's), was along the lines of "unless a seed fall into the ground and die it cannot bear much fruit." Sometimes characters can be worth more to us dead than they are alive. For instance Matthew's death transformed the relationship between Anne and Marilla, allowing Marilla to channel all the softer attributes that were once found in him. I wonder what grist might be obtained for the mill in terms of Al's character -- in terms of scenes between Al and his avunculars (Martin and Ruth) -- if Bert's character were to be sacrificed at this point?
I think you are right about the role Bert plays in this show. I wonder if that was the intention when the show began -- for Al to play such a prominent part or has the role grown and changed much like the prominence of the ME/LG romantic storyline? Chemistry cannot be denied and the actor who plays Al is terrific.
I don't think we'll see a unknown half-brother storyline. It is too much like a classic soap opera for ME/PB (and me too).
I think the death of Bert is a strong possibility. Like the death of AJ, it would strike at the emotional core of Al, and would provide an opportunity for Martin and Ruth to help. Might even be a reason to bring back Pauline? Could, however, the misdiagnosis that the medical advisor to the show hinted at, be about Bert? Might be interesting given Bert's hypochondria for him to be seriously ill.
Finally, I've been to Prince Edward Island where Anne of Green Gables is a cottage industry. It is every bit as gorgeous as described in the books. And it broke my heart when I read of Matthew's death. Much more than Dumbledore who I knew could show up in other forms