User blog comment:Argorrath/Treyarch posts a message of zombies; possibly hinting to a DLC 4/@comment-356904-20130801094138/@comment-1055346-20130801104329

It's just highly likely.

Maxis's note refers to "The Doctor" discovering "an element". Richtofen is called The Doctor on occassion and the element really can't be anything other than 115.

Everything else is just assumption, but it's a pretty good assumption. Treyarch even said pre-BOII release that the Original 4 would return in some way, and Vonderhaar has recently been hinting at it sporadically on twitter. It really doesn't seem that far fetched.