It is a shame we did not get to see a mega evolution of them when omega ruby and alpha sapphire came out, however, it might be worth rehashing the idea. I will be going through basic vision here for the mega evolutions, but open for discussion.
Start with Seviper:
Original Typing: Poison
Original ability: Shed skin/ Infiltrator
Original Base Stats:
73 Hp
100 Att
60 Def
100 Sp Att
60 Sp Def
65 Speed
Total 458
Mega Evolution:
New Typing: Poison-Dark
New Ability: Adaptability
New Base Stats:
83 Hp
125 Att
75 Def
125 Sp Att
75 Sp Def
75 Speed
Total 558
Overall Reasons:
With the new Dark typing Seviper is mainly only weak to ground type moves now by making it immune to Psychic. With the new Adaptability ability, it strengthens Seviper's STAB bonuses quite bit, opening it up to being a very viable option to dispatch fairy types. The stats were enhanced fairly equally across the board including adding some much needed defense and Special Defense while still keeping true to its main element of a strong and equal Attack and Special Attack.
Zangoose:
Original Typing - Normal
Original Ability- Immunity/Toxic Boost
Original Base Stats:
73 HP
115 Att
60 Def
60 Sp Att
60 Sp Def
90 Speed
Mega Zangoose:
New Typing: Normal-Dark
Ability: Toxic Boost
New Base Stats:
83 HP
150 Att
70 Def
60 Sp Att
70 Sp Def
125 Speed
Total 558
Overall Reasons:
First you will notice I went with a Normal-Dark instead of a Normal-Fighting type, while this now makes Zangoose more prone to Fighting type moves, it gives Zangoose another immunity, with his Normal Typing and Dark typing, effectively he is immune to Psychic and Ghost type moves and when poisoned is extremely powerful raising his base attack stat to 225 (450 with Facade)
Overall Zangoose does not get much in the way of much needed Defense, however this keeps with his uncanny high Speed and Attack stats, making him effectively faster than non-mega evolution Gengar and Alakazam (Ghost and Psychic types respectively) and with the power of virtually a train behind him.
Give me your thoughts and feedback. Thanks!