Let's see. The alternatives are
Leavanny: Excellent Pokemon, with access to STAB X - Scissor and Leaf Blade. With a Swords Dance up, you can sweep through the Dark and Psychic Elite 4. However, it is frail, so you might want to be careful using it. Ingame, it doesn't matter much, so if you're okay with waiting for it get to max happiness, then you're good to go.
A decent moveset would be: Swords Dance, X Scissor, Leaf Blade and Return, which works well with its evolution requirement.
Simisage: Strong, fast sweeper, but sadly doesn't get enough moves. Maybe once you get Surf and access to Energy Ball in Aspertia City, you can expect it to improve, but I don't think it's a good replacement.
Leafeon is found post ingame, so it's out of the box. Maractus and Sunflora are out because they're not as good either.
Roserade is a decent replacement, with access to STAB Energy Ball and Venoshock, so that should be good. Unfortunately, that places you at a disadvantage with the Psychic Elite 4. Further you need a Stone to evolve it, so you might have to search for that.
Sawsbuck is decent as a Physical attacker with access to STAB Return. Jump Kick + Horn Leech is a good combination, so you might want to try that out.
Ferrothorn and Tangrowth are better competitively than ingame, so you can skip them over. That leaves Carnivine, which is no better, so let's ignore it.
Finally, if you have no problem using a legendary, my personal favourite Unova Grass type Virizion is an excellent mixed sweeper with access to STAB Sacred Sword, STAB Giga Drain and X scissors, so it's definitely worth your consideration.
Overall, the best options would be: Leavanny/ Sawsbuck (Physical), Virizion (Mixed) or Roserade (Special).
Based on the rest of your team, you can choose any of the above.
