Mmm... Me thoughts are that he be gone for good, or will be brought back as a ghost (Like good ol' Constance) for the last battle with Roger. He will be defeated, have his power drained, (Used to bring back poor Walter) and finally be placed on one of the new Islands, diminished, a shadow of his former self. He'll be a weakened, but still powerful, boss; he'll be raising new minions to protect himself every few seconds. (Brings in the new enemies) His level will no longer be ??, since that shows unfathomable power, but rather an alert, something like this. !!
Please be aware that the above comments are probably not true and I never thought they were. Walter is either dead or dying, and I'm not ashamed to say I'm not all that dissapointed. (Fought him once; he actually wasn't that good with the cutlass...
) He was a character in a story, and therefore expendable in the overall plot. Now if they were to say Jack Sparrow had been killed, I'd be thinking that something's a little off here... I'm simply opening doors for speculation, like I tend to do.