Creek Henry, a .27 sounds cool and all, but I have to agree with tjh on the .20. Going back to my airgun days (even though I still shoot them).
Some of you may have a a Sheridan Blue Streak, they started way back, WAY before my 17 years of existence... I believe they were the first to introduce a .20 caliber pellet.
.177 pellets, offers a better ballistic coefficient than .22 caliber pellets of equal weight. The problem is that .177 is affected by wind way too much, considering it's weight. A good compromise is making a slightly larger projectile, while having the heavier weight of the .22 caliber pellet. This means little wind drift and less forward wind resistance (best of both worlds of the .177 and .22) and a pellet that can move further than the .22 with very similar knockdown power. I probably didn't explain this too well, but most people probably know about this anyway, or at least people who have experience with the Blue Streak.
I think this same concept could be revolutionary in the rimfire world. In fact, I wish they would come out with the .17, .20, and .22, but that would be a pipe dream, I'm sure. Either way, the .17HMR and .22WMR both seem to have their own markets and most people who own one, don't own the other (as a general rule), so coming out with a larger caliber round, they would still have a pretty large market.