I'll add my stinky little input on this cuz I like to give advice where I can.
Your team seems very well rounded in the typing department, if a little more focused on the offensive side. There's no problem with that, mind ya, but when it comes to offensive teams, what's most important is being able to kill whatever you're up against before they wall you or kill you instead.
First things first, I'm unsure of which server you're a part of (I'm a Diamond inhabitant), but if we're playing off of the National Dex ruleset, Mega Lucario is instantly crossed off for being banned.
Now, for a bit of an in-depth analysis. Right off the bat, you can't afford to have your Gyarados be your chosen Mega. Mega Gyarados is strong, sure, but its ability becomes kind of useless, and Gyarados's natural typing makes for a safe switch-in against Ground types or choice 'mons locked into a Ground type move. Additionally, I would arguably say Gardevoir would make a somewhat okay choice. Don't get me wrong, I love Mega Gardevoir, and there's a lot to love-- 165 base special attack, STAB Hyper Voice, wide movepool-- but Gardevoir, both normal and Mega, is super frail. Anything that outspeeds it will kill it, most likely. If you manage to survive, or happen to outspeed mostly everything on the opponents team, you should have no trouble letting your other party 'mons clean up what's left.
Sceptile is a strange case. It's extremely fast and has enough special attack to probably nuke most things, but its similarly frail like Gardevoir, and its movepool is insanely limited. We're talkin' Energy Ball / Dragon Pulse / Focus Blast / HP Ice, at best. You could make this work, though, because Mega Sceptile's typing pretty much only leaves it susceptible to Ice types and Flying types, but I'd say going with Sceptile is a very high-risk average-reward type of deal.
Now as for my personal recommendation, I'd have to go with Garchomp. I know I'm going against what Bornassassin recommended, but hear me out. Mega Garchomp is an incredibly powerful Mega option, with access to Swords Dance and Scale Shot. These two alone make Mega Garchomp into an unstoppable train of death if your opponent allows you to set it up carefully. Forcing a switch-out on your opponent is a great way to do this. Garchomp is also quite physically bulky (before the whole Scale Shot thing), which can help cover your Gardevoir, Chandelure, and Lucario if it comes down to it, and even acts as a safe switch-in against Electric types. Garchomp also has a very nice movepool, though I'd say Iron Head offers the best protection against Garchomp's counters.
Hope this helps a little bit, and I hope your team building goes smoothly. I'd say try each of the three out and see what ya like best.
