No pre-existing megas means no more Mega Swampert. With the literal perfect rain abuser out of the picture, there's now room to experiment with other Swift Swim mega designs.From slate 43 through slate 52, M4A will be focused on a Kalos dex format, which we will begin with no preexisting Mega Evolutions - not even official ones that were included in the original X and Y!

Mega Heliolisk
Type: Electric / Normal
Ability: Swift Swim
Stats:
HP: 62
Att: 65 (+10)
Def: 62 (+10)
SpA: 149 (+40)
SpD: 124 (+30)
Spe: 119 (+10)
The most striking ability of real-world basilisk lizards is their ability to run across the surface of water.

Incredible. The beauty and grace with which these magnificent creatures move is truly unmatched.
In addition to boosting its Speed to near-unreachable levels, Mega Heliolisk can make use of the rain to boost the accuracy of its Thunder, as well as power up its Surf coverage. It also has the option to replace Surf with Weather Ball, which is slightly stronger at the cost of not being able to be used effectively outside of rain. Additionally, its access to Volt Switch (and U-turn??? which it's been able to learn since its inception??? I never knew this thing could learn U-turn until now) allows it to draw in Grass-types and pivot out to a Hurricane user.
I considered going with Slush Rush to complete the quadfecta of weathers, but since Heliolisk already had strong anti-Ground coverage in Surf, I decided that Ice-type Weather Ball wasn't actually all that interesting. Maybe on a different Electric-type, but not Heliolisk.

Mega Wormadam (Sandy Cloak)
Type: Bug / Ground
Ability: Sand Force
Stats:
HP: 60
Att: 149 (+70)
Def: 135 (+30)
SpA: 59
SpD: 85
Spe: 36
Sandy Mega Wormadam has the potential to be something rather interesting: a physically-offensive Quiver Dance user. Sure, it won't get much use out of the Special Attack boost, but the Special Defense boost compliments its naturally high Defense, and after two Speed boosts it outspeeds every unboosted mon slower than Greninja.
Unfortunately, Wormadam's movepool is rather lacking. Earthquake is a strong STAB, but its best Rock move is Rock Blast, and it doesn't really learn any other useful physical moves besides Sucker Punch. It's still got a few different options for its last slot, though. Besides the aforementioned Sucker Punch, it can run Stealth Rock for some utility if it doesn't get the chance to set up, Substitute to help it set up, or Sandstorm as a sort of double dance. Though I think the most interesting move would be Hidden Power, fitting for the evolution of Burmy. Even with its low Special Attack, after a few boosts Hidden Power becomes strong enough to 2HKO problematic foes like Ferrothorn and Gliscor.

Mega Drifblim
Type: Ghost / Flying
Ability: Perish Body
Stats:
HP: 150
Att: 100 (+20)
Def: 104 (+60)
SpA: 110 (+20)
SpD: 34 (-20)
Spe: 100 (+20)
Mega Drifblim dominates any physical attacker that dare oppose it. Its titanic physical bulk makes any attack from them bounce off, it can burn them with Will-O-Wisp to make their attacks bounce off Mega Drifblim's allies, it can weaken them with Strength Sap and heal itself back up at the same time. Most devious of all, if the physical attacker ever touches it, Mega Drifblim puts them on a Perish Song timer, and it can even ensure they won't escape with Bind. What can be done to stop this defensive menace?
Well, just don't use a physical attacker. Not only is Mega Drifblim's special bulk significantly worse than its physical bulk, but none of its tools are effective against special attackers. They don't care about burns, they don't care about Strength Sap drops and give less healing in return, and they'll almost never activate Perish Body. Mega Drifblim can't even turn the tables on them with Calm Mind, since if it tries setting up a physical attacker can come in and force it out with its own Perish Body. Speaking of being forced out by Perish Body, Mega Drifblim is weak to Stealth Rock, meaning that whenever it switches out after Perish Bodying a physical attacker, it'll often take a 25% health penalty when it comes back in. It's also extremely vulnerable to U-turn despite its double resistance, since it'll be left with a countdown while the attacker gets away.