This performance isn't quite that rare. Most heavy combat helicopters (as opposed to lightly-armed recon helos) are also capable of aerobatic maneuvering.
The AH-64 Apache, the Mi-28, and the Eurocopter Tiger are all capable of +3 to +3.5 positive and -0.5 negative g. For the Ka-50, it's also +3.5 g; the negative load isn't stated in any reliable source, but might be in the -0.5 to -1 g range. Coaxials fail catastrophically if their load limits are exceeded, but perform well up to that limit.
If designing a helicopter specifically for aerobatics, one would want less weight and a smaller rotor diameter. So you'd have many shorter blades, to reduce blade flex. The tail rotor would have to be larger, to provide more control. The reason the Bo 105 only has 4 blades is cost.
A coaxial rotor helicopter would also need even more rotor separation than a combat or utility one, since they bend in opposite directions under load. Coaxials are potentially capable of more maneuvers, but their problem is also design cost and complexity - the transmission and the hub are elaborate and very highly loaded.