3rd Gen Sixth Addition to my Team?

Hi, I'm playing Emerald for the first time in years, and want your opinion on what my sixth Pokemon should be to help balance my team. I don't want you to say Rayquaza or a legendary, because I'd rather stick with regular Pokemon in my main party.

Current team:
Mightyena
Electrode
Sceptile
Lairon
Pelliper

I'm on the route to Lilycove City currently.

A Heracross wouldn't be a bad idea considering you lack a fighting type on your team. You can find it in the Safari Zone (Right around where you are) in the Acro Bike area.

Get a fighting-type. It will come in use in the Elite 4. (vs Dark and Ice)
A Heracross is mostly the best choice but if you dont like it, theres still Medicham and Machamp who are quite close.

Going to close as you got some suggestions already (I'll agree FWIW that Heracross is indeed cool). If you want more opinions on this, try the questions thread sticky in the In-game Team Help subsection of the Battle Centre section.