Gastrodon-West-Mega
Ability: Sticky Hold / Storm Drain / Sand Force ----> Slip 'n Slide (Water-type Motor Drive clone)
Typing:
/
Stats:
HP: 111 ----> 111
Attack: 83 ----> 105 (+22)
Defense: 68 ----> 77 (+9)
Special Attack: 92 ----> 115 (+23)
Special Defense: 82 ----> 82
Speed: 39 ----> 85 (+46)
Base Stat Total: 475 ----> 575
Movepool Additions: Hydro Pump, Aqua Jet
Design: Gastrodon-West oozes a purple liquid from its skin, which it makes its body very slippery. It slides around quickly while spraying and tackling opponents.
Description/Competitive: Gastrodon-West-Mega becomes much faster with boosts to its offensive stats. Its ability still allows it to absorb Water-type moves but boosts its speed instead. It gains access to some priority in Aqua Jet and a stronger Water STAB in Hydro Pump, allowing it to run mixed offensive sets. However, it retains its 4x Grass weakness and has no way of boosting its offenses.
Gastrodon-East-Mega
Ability: Sticky Hold / Storm Drain / Sand Force ----> Water Absorb
Typing:
/
Stats:
HP: 111 ----> 111
Attack: 83 ----> 83
Defense: 68 ----> 108 (+40)
Special Attack: 92 ----> 102 (+10)
Special Defense: 82 ----> 132 (+50)
Speed: 39 ----> 39
Base Stat Total: 475 ----> 575
Movepool Additions: Haze, Soak
Design: Gastrodon-East grows a shell made of water that blocks most attacks. When agitated or stressed, a bad-tasting purple substance fills up the water shell, which the Pokemon can use to spray at its opponents.
Description/Competitive: Gastrodon-East-Mega gains large boosts to its defenses which allow it to take hits much better. It also gains Haze which can be used to stop set up attackers that can't be hit with Clear Smog. Water Absorb is a better defensive ability than Storm Drain, allowing it to regain health rather than boost its special attack. While very bulky, Gastrodon-East-Mega is very slow and will likely have to take a hit before making a move. It also has an exploitable 4x Grass weakness, and its special attack stat leaves it somewhat passive.
Magcargo-Mega
Ability: Magma Armor / Flame Body / Weak Armor ----> Igneous Incarnate (Fire and Rock -type Volt Absorb variant)
Typing:
/
Stats:
HP: 60 ----> 60
Attack: 50 ----> 50
Defense: 120 ----> 160 (+40)
Special Attack: 90 ----> 115 (+25)
Special Defense: 80 ----> 100 (+20)
Speed: 30 ----> 45 (+15)
Base Stat Total: 430 ----> 530
Movepool Additions: Power Gem, Shore Up
Design: Magcargo's rocky shell expands to encase its entire body, leaving only holes for its eyes and mouth. Lava still leaks out of small cracks as it moves, leaving the battlefield a dangerous molten mess. It now moves around by rolling like a ball, and it apparently doesn't get dizzy.
Description/Competitive: Magcargo-Mega is much bulkier with higher special attack and speed. It gains Power Gem as a special Rock-type STAB move as well as Shore Up for extra recovery in the sand. Its ability allows it to recover health from opposing Fire and Rock -type moves, removing one of its weaknesses and giving it extra ways to regain health. However, Fire/Rock is a bad defensive typing with 4x weaknesses to Ground and Water while still being weak to Stealth Rock (which its ability doesn't protect it from) so its bulkiness can be circumvented. It also has lower special bulk and is still slow.
EDIT:
Sereg: Igneous Incarnate doesn't protect against damage from switching into Stealth Rock, but it does prevent the opponent from setting Stealth Rock in Mega Magcargo's face, healing it instead.
Swalot-Mega
Ability: Liquid Ooze / Sticky Hold / Gluttony ----> Contaminate (Poison-type -ate clone, yes it's been done before for other subs)
Typing:
Stats:
HP: 100 ----> 100
Attack: 73 ----> 110 (+37)
Defense: 83 ----> 96 (+13)
Special Attack: 73 ----> 110 (+37)
Special Defense: 83 ----> 96 (+13)
Speed: 55 ----> 55
Base Stat Total: 467 ----> 567
Movepool Additions: Hyper Voice, Recover
Design: Swalot becomes much larger with poisonous fluids constantly oozing from its body. It grows two more arms below its other arms which can all now extend from its body to grab things to put into its mouth.
Description/Competitive: Swalot-Mega gains significant boosts to its offenses with some increases to its defenses and Recover for reliable recovery. Its new ability gives it access to stronger or more reliable STAB moves in Contaminated Hyper Voices, Returns, and even Explosion. It has access to the elemental punches for physical coverage as well as some special coverage like Ice Beam for Ground-type opponents. Swalot-Mega gained no boosts to its speed stat so it remains slow and while its bulk and offenses are now usable, it can still be vulnerable to common Earthquakes that many Pokemon use as STABs or coverage moves.