The Espers of A Certain Magical Index/A Certain Scientific Railgun are interesting.
First of interest is the Level 5 Gemstone Esper Sogiita Gunha who violates the Laws of Physics heavily and has many unrelated powers.
I suspect that Esper Powers are what the Esper in particular expects to have which is why Level 5 Gemstone Esper has so many unique abilities since he got his Esper Powers from birth rather than through the Power Development Curriculum.
Another Gemstone Esper has the ability to draw Vampires and a Level 0 Esper Shundan Kimi summoned a Dragon that presumably represents the AIM Burst of a single Magic God(a Magician who got very powerful off of Magic Multi-Skill) as Touma who is infused with the will of all Magic Gods has multiple Dragons including Shundan Kimi’s Dragon.
Espers can technically gain any power they want and only require belief. Saten is only a Level 0 because she thinks she needs calculations and she is far too bad at math to preform the calculations she thinks she needs.
On to the next subject: Etsu Aihana the mysterious 6th Ranked Level 5 Esper. His/her identity and powers are unknown. Only the heads of Academy City and Misaki seem to know who Etsu Aihana is.
Since Misaki apparently knows Etsu Aihana it is probable that Etsu Aihana is Misaka 00000: Full Tuning. Full Tuning is the third oldest Misaka Clone(with Dolly 1 and Dolly 2 being older than her) and it would be ludicrous if Misaki wasn’t aware of her situation.
What the third oldest Clone has for her Esper Ability is unknown of course it could be that her ability is the result of Academy City testing to see if Esper Abilities are actually genetic or from personal preference(most likely the latter).