I have some obsessions when playing Pokemon, which are: No legendary, only one starter, always a ground type, only the Pokemon of the region I'm playing. (Like no Kanto or Johto Pokemon if playing RSE) And my Emerald team is:
Swampert - Quiet 252 HP 126 Attack 126 Special Attack (Only way to make use of EQ-Torrent both)
Earthquake - Ice Beam - Surf - Brick Break
Linoone - Jolly 252 Attack 252 Speed 4 HP
Belly Drum - Return (I can't get extremespeed, would be adamant and 252 HP otherwise) - Dig - Shadow Ball
Gardevoir - Timid 252 Special Attack - 252 Speed - 4 HP
Psychic - Thunderbolt - Ice Punch - Fire Punch (May replace one for Calm Mind)
2 Salamence
1 Adamant - 252 Attack - 252 HP - 4 Speed
Fly - Double Edge - Rock Slide - Earthquake (Not using against dragons obviously)
1 Modest - 252 Special Attack - 252 Speed - 4 HP
Dragon Claw - Hydro Pump - Flamethrower - Crunch
But I can't figure a sixth pokemon to fit this team. Can anyone help me out please...
I'm sorry if I'm wrong, but you don't seem to have any grass-type on your team, so first and foremost, I would suggest you to consider getting one. Although this is in-game, where type coverage like this don't exactly matter, it's still safe to keep it in-check, I believe.
If I were you, I would most likely go with Breloom; Shroomish is easy to find, train, and evolve, and once it's fully grown, it could cause some major destruction. I've tried it on my Emerald team before, and found it to be pretty marvelous. For move-set, I would recommend Spore, Focus Punch, Mach Punch, and perhaps Leech Seed.
Good luck!