Assuming every gym leader/E4 has a mega this time around, Highly doubtful but let us pretend, which Pokemon would be each ones megas?

Optional - What would their mega item be?

>lel bulba http://bulbapedia.bulbagarden.net/wiki/Hoenn_League

>Roxanne/Brawly/Wattson have no possible megas in ruby/sapphire

Using ruby/sapphire/emerald teams your choice

Emerald:
Wattson - Manectric
Flannery - Torkoal
Norman - Spinda
Winona - Tropius
Tate & Liza - Solrock & Lunatone (Assuming the game treats it as a multi battle)
Wallace - Milotic or Juan - Whiscash

E4
Sidney - Crawdaunt
Phoebe - Sableye
Glacia - Glalie
Drake - Flygon

Champion
If Steven - Mega Metagross
If Wallace - Mega Milotic