The main problem for me with them is the aesthetics of them.
Each core looks identical from above, other than a color and sticker change and they just aren't interesting enough compared to the heavily detailed attack and defense parts.
I know the reasoning behind the identical cores is for Striker stacking, but still, there really should be something about them that's more individual, and aestheticaly pleasing.
Differentiation in materials would be a good start in developing the series further, but I think that changing the shape of the parts is just as, if not even more vital...
Currently they seem to be one of two shapes circular, or trangular.
Some star shapes would be interesting, and extended wing or ramp sections would also be really nice to see instead of similarly shaped sections depicting skulls, waves, and eagles.
Don't get me wrong, it's cool that the parts are given character, but when they all make the same triangle or circle, then there really isn't much point.
The king of the pet peeves for me though is the flat stadiums.
This seriously limits the types of tips that you're going to want to use, and pretty much completely rules out the use of flat tips.
Even if the stadium had a very slight curvature to it, it'd be a definite improvement.