Senna has many choices when it comes to itemization:
There are a couple of niche builds as well, I've seen people mention about trying Grasp and build Locket for a Tank Senna build after Frostfire nerfs (although I've never tried it out myself) and also a moonstone into umbral glaive for the utility/healing
I would suggest you do some testing to see what suits your playstyle =)
Well i think it will be for a while to get the 2020 prestige skins in the new rotation shop and it is about 125~150 ME (12-15 gemstone) I have a feeling that the 2018 and 2019 prestige skins will go first then the 2020 pr. skins.
IMHO, it is better that you do the event pass and on top of that, they updated so that it is way less grindy
They said that in the game's patch notes, but not on their official patch notes page. I tested it on practice mode, but sadly it doesn't work. Only Q slows.
LoL client: "Passive - Absolution. New, Low visibility: Innate - Senna's basic attacks now slow enemies by 20% (+6% per 100 AP) (10% per 100 bonus AD) for 2 seconds"
But pach notes (https://www.leagueoflegends.com/en-us/news/game-updates/patch-12-2-notes/) Says: "NEW LOW VISIBILITY Now slows all enemies struck by 20% (+6% per 100 AP) (+10% per 100 bonus AD) for 2 seconds