No one's giving any more advice, so I'll get this off the unanswered list.
Your Flying coverage is fulfilled with Staraptor, one of the best Flying types in the game, so that's not an issue.
As you've expressed wanting to remove Zubat from your team, Roserade is a perfect Grass addition that overlaps types with none of your teammates. It's one of the strongest Grass types in Sinnoh, available early and able to carry it's weight through the entire game.
For your Ground type, Garchomp is arguably the best in the game. It comes fairly late, but it's easy to get up to speed with the Exp. Share. It's a powerful asset against the final stages of the game, and if you're able to get a Gible early on, the first part of the game is a cakewalk.
Alternatively, if you're not willing to wait that long, Gastrodon is a good Ground type. It overlaps types with Piplup, but it and Empoleon don't share any weaknesses. It's a solid Ground type substitute for Garchomp if you'd like Electric coverage early. Alternatively, Graveler can suffice until you get a Gabite.