My career has been guided by just the sort of unproven guess this year's question seeks.
My belief is that the potential for expanded communication between people far exceeds the potential both of language as we think of it (the stuff we say, read and write) and of all the other communication forms we already use.
Suppose for a moment that children in the future will grow up with an easy and intimate virtual reality technology and that their use of it will become focused on invention and design instead of the consumption of pre-created holo-video games, surround movies, or other content.
Maybe these future children will play virtual musical instrument-like things that cause simulated trees and spiders and seasons and odors and ecologies to spring up just as manipulating a pencil causes words to appear on a page. If people grew up with a virtuosic ability to improvise the contents of a shared virtual world, a new sort of communication might also appear.
It's barely possible to imagine what a "reality conversation" would be like. Each person would be changing the shared world at the speed of language, all at once, an image that suggests chaos, but often there would be a coherence, which would indicate meaning. A kid becomes a monster, eats his little brother, who becomes a vitriolic turd, and so on.
This is what I've called "Post-symbolic communication," though really it won't exist in isolation of or in opposition to symbolic communication techniques. It will be something different, however, and will expand what people can mean to each other.
Post-symbolic communication will be like a shared, waking state, intentional dream. Instead of the word "house", you will express a particular house and be able to walk into it, and instead of the category "house" you will peer into an apparently small bucket that is big enough inside to hold all the universe's houses so you can assess what they have in common directly. It will be a fluid form of experiential concreteness providing similar but divergent expressive power to that of abstraction.
Why care? The acquisition of post-symbolic communication will be a centuries-long adventure, an expansion of meaning, something beautiful, and a way to seek cool, advanced technology that focuses on connection instead of mere power. It will be a form of beauty which also enhances survivability; Since the drive for "cool tech" is unstoppable, the invention of provocative cool tech that is lovely enough to seduce the attention of young smart men away from arms races is a prerequisite to the survival of the species.
Some of the examples above (houses, spiders) are of people improvising the external environment, but post-symbolic communication might typically look a lot more like people morphing themselves into varied forms. Experiments have already been conducted with kids wearing special body suits and goggles "turning into" triangles to learn trigonometry, or molecules to learn chemistry.
It's not only the narcissism of the young (and not so young) human mind or the primality of the control of one's own body that makes self-transformation compelling. Evolution, as generous as she ended up being with us humans, was stingy with potential means of expression. Compare us with the mimic octopus which can morph into all sorts of creatures and objects, and can animate its skin. An advanced civilization of cephalopods might develop words as we know them, but probably only as an adjunct to a natural form of post-symbolic communication.
We humans can control precious little of the world with enough agility to keep up with our thoughts and feelings. The fingers and the tongue are the about it. Symbols as we know them in language are a trick, or what programmers call a "hack," that expands the power of little appendage wiggles to refer to all that we can't instantly become or create. Another belief: The tongue that can speak could also someday control fantastic forms beyond our current imaginings. (Some early experiments along these lines have been done, using ultrasound sensing through the cheek. and the results are at least not terrible.)
While we're confessing unprovable beliefs, here's another one: The study of the genetic components of pecking order behavior, group belief cues, and clan identification leading to inter-clan hostility will be the core of psychology and sociology for the next few generations, and it will turn out we can't turn off or control these elements of human character without losing other qualities we love, like creativity. If this dark guess is correct, then the means to survival is to create societies with a huge variety of paths to success and a multitude of overlapping, intertwined clans and pecking orders, so that everyone can be a winner from equally valid individual perspectives. When the American experiment has worked best, it has approximated this level of variety. The virtual worlds of post-symbolic communication can provide the highest level of variety to satisfy the dangerous psychic inheritance I'm guessing we suffer as a species.
Implicit in the futures I am imagining here is a solution to the software crisis. If children are breathing out fully realized creatures and skies just as they form sentences today, there must be software present which isn't crashing and is marvelously flexible and responsive, yet free of limiting pre-conceptions, which would revive symbolism. Can such software exist? Ah! Another belief! My guess is it can exist, but not anytime soon. The only two good examples of software we have at this time are evolution and the brain, and they both are quite good, so why not be encouraged?
The beliefs I chose for this response are not fundamentally untestable. They might be tested someday, perhaps in a few centuries. It's not impossible that medical progress could keep me alive long enough to participate in testing them, so strictly speaking I can't guarantee that I can't ever prove these beliefs to be true.
There are not too many potential beliefs that could really never be tested by anyone ever.
Consciousness, meaning, truth, and free will and their endless permutations just about complete the list. The reason philosophy is so much harder to talk about than science is that there's so little to talk about. It quickly becomes almost impossible to distinguish repetition from resonance.
Proposals like post-symbolic communication, however, frame questions about meaning that are small enough to be fresh and useful. Am I right that there can be meaning outside of words, or are the word-as-center-of-meaning folks correct?