I don't see a problem of an HF for a critical bug like that, people will have to upgrade or the node stops. Hard to tell the block size increase required by that time. I think blocksize should be an algo that increases the limit a small percentage if blocks are full for a long period of time.
Discussion
See the “should be” part. There will be numerous proposals that will try to add in stuff on top of fixing the bug, which could splinter the hash. Looks like it’ll be an ugly affair.
I don't think other stuff should be added. If there is a problem with the blocksize I think we hit that wall well before that date.