If we're wanting to replace Tropius with some other grass-type, I feel Breloom would be a good path to take.
Breloom's got good Attack and Speed stats, as well as access to a wide range of moves which we might find to be quite helpful, such as Spore, Leech Seed, Seed Bomb, Mach Punch, Focus Punch, and Substitute (for stalling purposes). I have it on my team right now in Omega Ruby, and it performs quite wonderfully, without any question.
Alongside, another suggestion would be Roserade. Although I haven't used it much myself, it does seem to have a decent amount of Special Attack stat and whatnot, and with that, I believe it would be able to make a valuable crew member.