What other Pokemon should I add to my Emerald team?

You need an electric pokemon - a water pokemon would be effective against most of your members.

Try adding an electric type, like Magnemite, Electrike or Voltorb.

They're around Mauville so it should be easy for you to catch one, and I always use Thunder Wave to help catch any other Pokemon.

Also, I completed my Ruby game with a Swellow just because it's that good. It even reached lvl100 before my starter o.o

Flying types really help out a lot, especially if they're also normal types (good against ghosts if you use other type moves)

I can't really remember which gym leaders there are after Mauville... but I know that you must always have a well rounded team just in case.

Hope this helped!

take out geodude (uc alredy aron and golem sucks)

get a zigzagoon for free items and as a HM slave

get ralts if possible it not,

get jolteon if possible, if not, manectric is fine

can't see why you didn't get that tailow...if you don't want one, get a zubat (crobat pretty good) then get bagon in the yellow cave (best no legend in the game)

cant think of much else...

A good pokemon to have is going to be Swellow (evo of Tailow, if you don't know). Simply because Swellow/Tailow can learn Fly and is very useful against fighting-type pokemon.

I would keep combusken and aron as they are OP once they evolve and you should add manetric, rayquaza, wailord and salamence