I dont know that there are any gaps in ammo when it comes to hunters / hunting.. theres something already out there that is more than capable of humanely taking everything from a field mouse to a bull elephant, thats been doing it exceptionally well for God only knows how long already at pretty much any reasonable/ethical distance....
Sure we could always find a way to squeeze another 50 FPS out of a 300 magnum.. or figure out how to make an "improved" 416 that will accomplish 2400 fps with a 400gr projectile 5% more efficiently than the last 416 developed.. but.. those improvements at this point are somewhat immaterial I think..
Dont get me wrong.. I have toyed with wildcats, new cartridges, etc.. most of my life.. its fun trying new things, playing with something no one else has, etc..etc..
but is 300 PRC really filling a "gap" that the 300 WM or 300 weatherby or 300 H&H wasnt already filling? or is the latest/greatest 6.5 really an improvement over the 6.5 sweede or the 6.5 creed or fill a "gap" that was out there?
About the only exception I see (that I think has largely been filled already, but because politicians are going to politician, will probably have to be filled again at some point) is the industrys response to truly ridiculous legislation in some states.. like "straight walled cartridges" being required.. perhaps 350 legend, 450 bushmaster, etc.. of recent fame helped fill some gaps between lever action pistol caliber carbines (357 mag, 44 mag, etc) and the 45-70 that folks had previously been using to meet those states requirements..
If we transition the discussion to extreme long range shooting, or extreme precision shooting (benchrest, PRS, F1, etc..etc..) then theres probably still a lot of room for improvements out there.. in those sports the tiniest of fractions of group size improvement can make the difference in a win or loss.. a hit or a miss.. etc... so if you could figure out how to fly flatter, straighter, more consistently, etc by even the smallest measure, people involved in those sports would be all over it..