One cool feature could be its speed. Mega Pikachu might be able to move at lightning - fast speeds, making it almost impossible for opponents to land a hit. It could zip around the battlefield, leaving a trail of electricity in its wake.
Another feature could be its new type of electric attack. Instead of just Thunderbolt, it might develop an attack like 'Electro Nova', which explodes in a huge burst of electricity, affecting a wide area. In fanfiction, this could be described in great detail, like the bright blue - white light blinding opponents and the deafening crackle of the electricity.
The appearance of Mega Pikachu can also be a really cool feature. It could have glowing blue markings all over its body that not only look cool but also serve a purpose. Maybe these markings can absorb and store extra electricity, allowing Mega Pikachu to unleash a super - powerful attack when it has enough energy stored up.