So, recently, I've been looking to change up my usual Janna build. Normally I start off with Ancient Coin and upgrade my boots to Boots of Mobility, which are the 2 things I want to change.
I've been thinking of starting with Spellthief's instead, for the mana regeneration and ability power, and even though the Janna mini rework back in S7 pushed her into a enchanter/poke support role similar to Karma, she isn't really the best at actually poking, aside from spamming her W. Not to mention with Ancient Gold once you level it up it becomes incredibly easy to get your wards before 8 minutes, but the fact that you have pick up the coin can make it somewhat dangerous due to Janna's general squishiness, even more so when against supports like Morgana or Bliztcrank. With the new changes in the next patch getting rid of the Bandit passive on Ancient Coin line, I'm leaning more toward Spellthief's.
When it comes to boots, I almost always go Boots of Mobility because it makes her just ridiculous fast and get anywhere in the map in less than 20 seconds. However, with her passive and the passive on her W, Boots of Mobility isn't needed as much on her as it would be on other supports like Nami and Soraka who suffer from having little to no movement speed. My new options I'm looking into are Sorcerer's Shoes, for the magic penetration, and Boots of Swiftness, due to it giving more movement speed then anyother boots, not counting mobis, and the slow resist. I like Sorcerer's becaus the magic penetration, albeit small, would allow her to deal a little bit more damage in lane. However, Boots of Swiftness offers more movement speed, which in turn also increases her damage because of her passive, plus the slow resist would only make it harder for the enemy to lock me down. I usually take the Celerity rune on Janna, so any movement speed is appreciated, which causes me to lean in favor of the Boots of Swiftness.