When comparing BW with Pyro and UC, it is quite noticable that BW has alot less synergy between ranged and melee than the others. UC being having obviously the highest with overcharge increasing melee damage and then using your active ability while in melee to clear overcharge so you can spam more ranged so you can get your overcharge up again… you get the idea.
Pyro is in a similar position, higher crits mean also more damage in melee and SS is very good in balancing out the overcharge speed penalty. She also gets attack speed talents, one just passive and another being tied to her passive ability.
And this is where I think BW drops the ball: While she has some sort of synergy between ranged and melee with being able to select more stamina, conditional block cost reduction, lingering flame also applying to melee burn damage and World Aflame obviously being triggered more when you are in melee, the synergy-factor is very low. Pyro can at least somehwat gear towards her abilitiy boosting her melee prowess (attack speed, more crits and even temp health generation all increase your survivability and damage output in melee) and more importantly, her passive ability is almost equally as useful for ranged as it is for melee.
BW does not have that. She a.) needs to get those bonuses via talents, b.) they are comparetively situational and conditional, with you having alot less controll over when they apply (when your Tranquility is on cooldown, you won’t get the BCR bonus, World Aflame needs enemies nearby, etc.) and c.) they are alot less tied to her abilities that boost her ranged.
Which is interesting because both Pyro and UC have a rather elaborate focus on this synergy and it is surprising they did nothing with that on BW and her reckless haste ability and only to some degree with her Tranquility.
Thus, my suggestion would be to strengthen this bond and give her more tools to play with. This would also be rather easy since her level 10 talents are rather weak, Lingering Flames being the stronges pick but it is very build dependant. World Aflame is unreliable damage bonus you can hardly build around when considering break points and Molten Skin is just rubbish.
But I think it would be rather easy to repurpose them into something useful by tying the level 10 talents to her reckless haste while at the same time improving her melee and ranged synergy.
Change Molten Skin to a passive damage reduction bonus tied to reckless haste, which would also keep the “spirit” of the the talent of Sienna’s Firemagic providing her with defensive prowess. Molten Skin could provide somewhere between 10% and 15% of damage reduction per stack of restless haste. So Sienna has an easier time throwing herself into melee when her overcharge is high and it would also counteract the mobility penalty provided by high overcharge by allowing her to eat a couple more hits. Sienna throws spells, overcharge gets higher, spells start charging faster, overcharge is higher, now Sienna needs to go into melee and is a little harder to kill.
World Aflame could be in a similar spot, but one could be a little more creative here. Of course, just switching it out with a flat out power bonus of 5% per stack of reckless haste might be a thing. A more creative thing would be to give Sienna an Aura that deals ticks of fire damage to nearby enemies and both radius and damage increase with reckless haste. It would also be in tune with the theme of having one offensive, one defensive and one utility option.
Lingering Flames could also be modified in a way that is linked to reckless haste. While keeping the “target burns forever but only one stack” thing, it could also increase burn damage by 10% per reckless haste stack. This would benefit her ranged, too, but also the burn damage of her melee weapons.
Just from the top of my head, but I feel there is alot more potential to BW, especially when considering there should be incentive for here to go into melee.