There are plenty of reasons creatures are the single most dominant creature type in most Magic sets. They have an inherent coolness, more player psychographics love or really like creatures than dislike them, and the combat phase is simply a lot of fun. But one of those reasons is purely a design conceit: It's easy to make a variety of creatures that feel and play different with a handful of evergreen keyword mechanics by varying their power and toughness. In that sense, while the design space for creatures is not infinite (it's bounded by what can be considered reasonable mana costs and the limitations of life totals), it's certainly bigger than we can imagine. Recently, Mark Rosewater answered two queries regarding odd, rarely-seen power and toughness numbers on his Tumblr blog, which give us some glimpses into the future of Magic's creatures.
Negative power and toughness: Yes, some creatures are so alien that they can't survive without pump. Zero-toughness creatures, such as Force of Savagery, are a near-example, but what's really being talked about here is negative numbers in the power and toughness box. The first such creature in Magic history was Spinal Parasite, which was not well received (it comes out as a 4/4 for WUBRG at best, for crying out loud) due to being generally terrible despite its interesting design. That alone makes it seem as though negative power and toughness was a one-off experiment that we'll probably not see again. Char-Rumbler, though, which came along years later, is considered much more fun to use given its double strike, built-in pump ability, and synergy with a lot of power-pumping cards. Mark Rosewater seems to believe that there will be a design application that calls for negative power and toughness somewhere down the road; when asked by Tumblr user radioactivestardust at the chances of seeing such a thing in future sets, he said:
I’ll say a 4 [on the Storm Scale]. Odds are we’ll do one again when we find a good reason.
+1/+1-counter keywords are a staple of current set design, and I wouldn't be surprised if there were a negative-P/T creature with, say, devour if that keyword returns.
Huge (20/20 and higher) power and toughness: Yes, I'm talking about player killers. In an ordinary set, creatures only go up to perhaps 10/10 to 13/13, but way back in Unglued R&D printed a 99/99 (!!!), B.F.M. (Big Furry Monster) as a joke, notably including what may be the first anti-reanimation clause in Magic. So yes, sometimes Un-set mechanics do creep into "legitimate" Magic, and then a fun time is had by all - see The Cheese Stands Alone/Barren Glory and Look at Me, I'm the DCI/Nevermore. The concept of an unfathomably big creature wormed its way in with Coldsnap's Dark Depths, which technically only produces a giant creature token. So there's already a bit of precedent, and when Rosewater was asked by Tumblr user apenpaap "Do you think a 20/20 creature will ever be printed, or is the opportunity for combo shenanigans (like Fling) that kill an opponent in one hit too great to make it?", he answered,
We’ve printed a 99/99 creature so I’m pretty sure one day we’ll print a 20/20. It will have some safety measure on it so abusing it won’t be so easy.
A creature that big doesn't sound plausible in a normal Magic set... but it seems like just the thing for Commander. Just as a speculative example:
Magnosus Omnidroon UUUURRRRGGGG
Legendary Creature - Treefolk Kraken (Mythic)
You can't cast spells.
Magnosus' galaxy-spanning tentacles arranged the order of the multiverse. If he so wishes, they can just as easily tear it apart.
Granted, this might need a somewhat different drawback, including maybe an anti-reanimation clause, but I like this as the culmination of a tradition of huge fatties with crippling drawbacks in blue, red, and green. A creature that's "officially the biggest thing in serious Magic, and will be, forever - no ties" is pure unadulterated wow factor. 21/21 is also a great number for many reasons - association with blackjack wins, the fact that this is "bigger than a player," and that 21 damage is exactly the number that a single EDH commander needs to deal to a player to cause them to lose.
What do you think about the future of really big and really small creatures? Let me know in the comments.