I have a PBE account as well. As soon as he comes out I'm going to try my best to bug the crap out of him so I can report anything.
Example:
What happens if you put his portal on the outside of an enemy base gate? Will they be locked from exiting their base from there until your portal disappears?
If a Master Yi/Fiora Q's/Ults before the activation of Tempered Fate, will they come out of their animation and be frozen, or be able to still move since they were invulnerable?
Here's a biggie. Can you Zhonya's to avoid the forced Zhonya?
Can his gates be used to cross the large gap from the Dominion base platform to the lane ahead? (Same with Base>Jungle gap in TT)
If Bard uses Tempered Fate while someone is recovering from Zilean Ult/Guardian Angel/Aatrox/Anivia/Zac Passives, what happens? ESPECIALLY with Anivia and Zac.
I notice there is a travel time when going through portals. Are you invulnerable during this time? (Could you use it to dodge Requiem or something)
If Lee Sin hits someone with a Q, then they go through a portal, will he go through the portal with second Q or just dash straight to them?
What the heck happens if Bard uses Tempered Fate on a Yorick-Revived champion?
What happens if you use Tempered Fate on ANY champion that is currently being displaced? (Being popped up, knocked aside, Gragas ulted, etc.)
What happens if you ult someone Polymorphed by Lulu? Are they stuck that way?
What happens if your ult hits an undead Kog'maw?
Pantheon ults. Bard ults either the jump location or the landing location. What in the world happens?
What happens if you use Tempered Fate on any stealthed champions?
What happens if (hard to replicate but it can happen) you ult a Rengar mid-jump? Or any champion (Kha'zix, Tristana, Aatrox, etc.) mid jump?
What happens if you use Tempered Fate on people currently in Nami's Q?
Can Gangplank cure himself of the Zhonya's, or no since it is not a CC?
What happens if you use Tempered Fate on Zac mid-ult?
What happens if you use Tempered Fate on Lissandra while she is locked in her own ult? Considering hers is a stasis as well?
What happens if you ult Kalista while her soulbound is held by Fate's Call?
What happens if you somehow manage to catch Rek'sai mid-travel during her ult or tunnel passage?
Can champions that ignore CC (Sion with ult, Vi with ult, Nocturne with ult) be hit by Tempered Fate, as it is not CC, but stasis?
What happens if Soraka uses Wish while another is in stasis? (kinda general but curious)
Will spell shields/Morgana's Black shield stop Tempered Fate?
So. Many. Things. To. Test.