Hmm...partners for Roserade...
-As pointed out already, Rotom-W is a perfect partner, and Rotom-H is also pretty solid.
-In pure typing matters, Houndoom is also actually rather nice, for those seeking a possible partner for mega-houndoom. Roserade resists fighting and water moves, flash fire houndoom has immunity to psychic and fire attacks, while also having an ice resistance. And Roserade can actually use Sun for 100BP fire-type weather ball, for those wanting to risk playing with Solar Power on Mega-doom.
-Mandibuzz is not a terrible partner either, with the ability to phaze for hazard damage, come in freely on psychic and ground attacks, and be more physically bulky as a counterpart to roserade's special bulk. The two do share an ice weakness though, which can be quite dangerous.
-Sableye is not bad to handle the physical half of the equation, with prankster burns and recover to stall out and force switches and hazard damage.
-Chandelure is also a fun choice, able to come in freely on fire attacks with flash fire, does a lot of damage, and has the bonus of being a spinblocker.
-Also likes various water-types, naturally. Starmie and Slowbro resist fire, ice, and psychic attacks, greninja might appreciate spikes from somewhere else if you want it more offensive, or just want to get more layers down faster. Any roserade without synthesis doesn't care about rain either, making that another possible niche.
-Steel types are not bad, but it leads to shared fire weaknesses, so if you pair them, you def need another teammate that can beat the heat.