I know I said I'd post this to you wall, but meh. I just woke up from a 12-hour sleep, so I'm a bit groggy xP
The key to using niche Megas like Aerodactyl is that you have to be able to justify using them over something else. It has to be doing something that no other Pokémon can do. And, most importantly, Megas like this should support the team. Building one around them usually ends in failure because they become too reliant on their teammates to accomplish anything.
For example, I love Mega Beedrill. He is my favorite Mega Evolution, and I wanted to use him on a team that worked. In the [paraphrased] words of the great Nugget Bridge user Hibiki, "I built a team that worked both with and without Alakazam." This inspired me to find something that could benefit from Beedrill's A) lightning-fast U-Turn, and B) ability to check Grass-, Fairy- and Steel-types. Grass and Steel types are usually a big problem for Rain teams. My favorite Rain sweeper, Kingdra, hates Fairies. Rain also dislikes Pokémon like Charizard Y and Tyyranitar who override my weather with their own. Beedrill outspeeds both and can U-Turn into Politoed, resetting the weather in my favor. So Beedrill (in theory) can be a help to Rain teams. In turn, Beedrill benefits from the Rain. Most players are reluctant to bring their Fire Pokémon, Landorus, and Terrakion to a Rain match because they have a bad matchup against that archetype. This gives Beedrill a lot of breathing room because there is a smaller chance of his main checks showing up. Even if Lando & co. do appear, most of the Rain-abusing Pokémon can take them down easily (I have four Landorus checks). This is my team, only one EV spread away from testing.
But enough of my Beedrill glorification. We want to figure out what can support Mega Aerodactyl as well as benefit from him. Yesterday you brought up maybe using him with a Sand Stream Pokémon. There is a popular archetype going around right now – you may have already heard of it – called Japan Sand. It consists of:
Later, Aaron "Cybertron" Zheng modified it a bit.
For the June International Challenge, a Nugget Bridge user named gingeruprising used Mega Pinsir on this team, substituting it for Mega Salamence. I though that the same thing could be done for Mega Aerodactyl. It would proably look something like this:
Tyranitar, Excadrill, and Amoonguss all remain the same and do the same jobs. Aerodactyl jumps in place of both Salamence for Flying offenses and Aegislash for Wide Guard support. Now you have three Water weaknesses; Gastrodon helps with that by providing Storm Drain support. Finally, not only are you weak to Intimidate but you have a spot open for a Steel-type. Bisharp looks good here, with Defiant to punish Intimidate and fire off +1 Sucker Punches.
There are obviously other Pokémon to try using, like Milotic or Heatran, or other archetypes to use Mega Aerodactyl on, but otherwise I think that Mega Aerodactyl feel right at home on a Sand team.
TL;DR Mega Aerodactyl likes Sand. Good partners are Tyranitar, Excadrill, Amoonguss, Gastrodon, and Bisharp.
TL;DR The Mega and the team must mutually benefit from one another.