Note: I am suggesting these under the assumption that you will be playing the game normally and not trading over a team of 5 IV all-stars.
If we're going to suggest team members based off of the original R/S/E PokéDex, then I wouldn't be so quick to suggest either Metagross or Salamence. You get them at the end of the game, and by then they won't serve any real purpose. They will be under-leveled, plus a real hassle to train. If you want a reliable steel type, you might have to go with Aggron here. You can get Aron pretty early in the game; it can be found in Granite Cave as soon as you get to Dewford.
The best alternative to Salamence would most likely be Flygon, though you might find success with Altaria should you opt to use its mega form. Swablu isn't found too late in the game either, and it evolves into Altaria at level 35, which is far more reasonable than level 50 for Salamence. Trapinch will have evolved into Flygon by level 45, and though it might not be as powerful as Salamence, it learns Dragon Claw 6 levels earlier, and gets a STAB Earthquake which should be more than enough to handle anything you will be battling in-game.
I'm not a big fan of Sableye, but it isn't as if any of the ghost type Pokémon native to Hoenn are known for being powerhouses in-game. If you want an offensive ghost, you will have to make what you can out of a Banette. It has a pretty nice base 115 attack. While not being the quickest thing around, it can sort of compensate for this with moves like Shadow Sneak and Sucker Punch, both of which it learns via level up. It also has the ability to mega evolve, which gives it slightly better defenses and a pretty high base 165 attack. It also gets Prankster upon mega evolution, so you could use this to throw around Wil-O-Wisp. It learns the move naturally, and it could help it work around that mediocre physical bulk. If you want something a bit bulkier, you have the Duskull family, though you won't be OHKOing anything unless you're several levels higher than it because Dusclop's offensive stats and offensive move pool are nothing sort of abysmal.
I'd also suggest that you use Manectric. You can get Electrike pretty early on, and mono-electric coverage is surprisingly usable in-game. It learns both Flamethrower and Overheat via TM, both of which are pretty good options for taking out grass type Pokémon, and things that will resist your electric STAB like Magneton. It sort of sucks that it doesn't learn a grass or ice type move outside of Hidden Power, but maybe if you're lucky enough you can catch (or breed if you want to) one with either of those. It can also mega evolve.
Gardevoir or Gallade are both pretty good options. You can get Ralts very early on, though finding and training it can be a real pain. Gardevoir gets Moonblast and Psychic via level up, and can be taught Shadow Ball, Thunderbolt, Focus Blast, and Energy Ball (I mean if you want to use this lol) via TM. Gallade learns Swords Dance, Close Combat, Psycho Cut, Leaf Blade, and Night Slash via level up, and can be taught Stone Edge and Poison Jab (for lame fairy types) via TM. Both Pokémon are capable of mega evolution.
A few more notable Pokémon are Sharpedo, Camerupt, Zangoose, Seviper, Mawile, Ludicolo, and Shiftry. I hope this helps, and good luck with the team.
