As someone who wants Elise buffs, the PBE changes are a bad idea
The problem with Elise while she was too powerful was how much of her power came in raw stats. Basically everything strong about her was for free - high base damages on Q, high base % damage on Q (no need at all to build AP for the scaling, have fun going full tank with no loss of damage!), free tank stats, high base on-hit damage, high human W damage, the list just goes on. Everything Riot has nerfed about her in the past was stuff she was given for free.
Now they are increasing the missile speed on Cocoon and the movement speed in Spider Form to the game's highest: 355. Now these aren't big buffs, I don't think they'll make her broken, but... Ask yourself - what does an Elise player need to take advantage of either of those two things? Nothing. They don't need to build any items, don't need to make any choice (say, between damage and tankiness), and Cocoon which is her core skill shot is now easier to hit again. Less risk, same reward.
This is just tuning her base stats to be good again. Last time her base stats were good people screamed for nerfs. How about something that actually requires trade-offs and maybe makes her more satisfying to play? Please just alter her damage already. Cut bases and buff scaling so she's encouraged to build damage and won't fall off horribly when she does, or something.
Just do anything other than giving the queen of base stats even better base stats. A viable build path that she's encouraged to take and actually serves her well would be a great start because "tanky magic pen" isn't really doing the job right now.