Your team needs a way to deal with Water. The later game of Sinnoh has a lot of Water types in it, so I would definitely recommend getting a Grass or Electric type. (In my diamond playthrough I had to get Shock Wave on my Dialga because I had nothing for Water, and Cynthia's Milotic suuuuucked.)
I would recommend Abomasnow but you already have an Ice type so next Pokemon.
Luxray is a really cool option. Luxray is not a Pokemon used often in competitive because of its mediocre stats, but it has an amazing attack stat of 120. Definitely a nice change of pace.
If you want a Grass type, there aren't that many good options after Abomasnow. If you really want a change of pace, you should got for Tropius, but that would also be a third flying type. I think two solid options would be Roserade or Carnivine. Roserade is kind of a stereotypical grass type to use in a Sinnoh playthrough, so I would pick Carnivine. It doesn't have the best stats, but it's pretty unique, and definitely a fun Pokemon to use. It has an Attack stat of 100 and Sp. Atk of 90, as well as mediocre defenses. Roserade on the other hand, has a nice advantage of the secondary Poison typing, as well as a good movepool.
Hope I helped!