If you gonna make a magician concept, make her a magician. No need to make her an iron balloon who excel in physical stats also. Optimize her role as elemental wizard. Stop making Prometheus, Zeus a personality of their own. Make her literally her personification. Every time prometheus talks, it's Big Mom talking. No need to make the fear-based soul-sucking ability, but remove the restriction of her elemental control. Make her in absolute authority and over any element she managed to give soul to. Giving a soul to fire = greatly amplified resistance and even healing against fire-based, heck even heat-based attack.
Next level, make soul-link ability. Like if the enemy is a logia, BM giving soul to a piece of that logia body would gain partial control of the body part that is given BM soul.
I think the problem here was that very early on, Oda belatedly recognised that the Soul Fruit is too versatile and capable of varied powersets. On top of the powers that have been portrayed Oda could have gone really wild and added things like:
- Strength enhancement = Draw the lifeforce of others and add to her own strength, increasing it beyond limits
- Physical enhancement = Draw the lifeforce of others and and increase her own size and power
- Life enhancement = Draw upon the Lifeforce of others and add to her own, either giving her a longer potentially endless lifespan or increasing her lifefroce reservoir so she doesn't die from fatal attacks
- Defence enhancement = Draw upon the Life force of others and use it like a forcefield above her skin or underenath it making her invulnerable
- Liche/Horcrux = put her soul or bits of it into others (e.g her homies) making her immortal. The only way to kill her is to kill her homies (no easy feat either)
and so on and so forth
and these are just generic soul powers off the top of my head.
But Oda didn't want to make BM potentially the strongest, invulnerable, immortal, unkillable villain
that is wholly dependent on her DF. She'd be similar to someone like pre-TS Croc, or Enel. He wanted BM to be an all-rounder, and not just specialise in one thing. Just having her be a magician wasn't enough. That's why he decided to give her super strength, top-tier haki and seeming invulnerability from birth, wholly separate from her acquiring the DF.
The thing that really irks me is that Oda never fully explained where her super strength and invulnerability come from. We can only speculate it's some form of natural born durability, perhaps 'Tekkai' but completely instinctive.