There are so many fully-evolved Pokémon that are underused and cast aside due to their mediocre stats. Since Mega Evolution was introduced in Generation VI, some Pokémon were given a chance to shine through the power of Mega Stones. I believe that no Pokémon should be forgotten, abandoned, ignored, or cast aside. Many Pokémon really deserve a Mega Evolution.
Here is my example and idea for a possible Mega:
I think Electrode deserves to have a Mega Evolution; it is my old Gen I favourite, and I believe it is sadly underrated. Its mediocre Attack/Sp.Attack stats prevent it from really taking advantage of its incredible Speed stat. Despite Explosion being Electrode's "signature" move, its poor Attack stat does not provide it with sufficient power. Its Sp.Attack is average at best, but it still isn't powerful enough. I believe Electrode deserves a Sp.Attack boost, and perhaps a new explosive Electric-type move that is Special rather than Physical. The Levitate ability would benefit Electrode immensely.
Some believe that Electrode has a boring design that cannot be improved on, but I disagree. I picture a Mega Electrode looking something like a Plasma Globe. Its spherical body would become translucent and larger (due to it being full to bursting with electricity), and inside its spherical body, a luminous blue ball of plasma-like energy could be seen, surging with internal sparks of cyan-bluish plasma. Some of these internal sparks would leak out, surrounding Mega Electrode in an aura of crackling sparks. Its eyes would be wider and glowing red, and it would have a crazed electrified, sharp-toothed grin on its face, as it is surging wildly with overflowing electricity, causing it to become manic. After all, it is said in Electrode's PokéDex entries that Electrode often absorbs too much electricity. What if Electrode could contain and use its overflowing electrical power without exploding all the time? Now that would be quite awesome, in my opinion.
Post your ideas for possible Mega Evolutions in the comments. Which underused Pokémon do you think deserve to join the ranks of the Megas?