With all that Flash in the air, a lot of people are siding Leyline of the Void; how do you think this will affect Threshold ? In particular, do you think it would still be a good choice in the meta ?
Yes, the deck is still viable. Only IGGy runs Leyline maindeck; a lot of other decks will be running them but they still need to make room, have in one their opening draw (40%) and still have an otherwise keepable hand. So, the fear of Leyline is more on the order of Fear of the Big Bad Circle.

Bardo, what is you current post-FS UGW-Thresh list ? (Thanks in advance). Are Mongeese still relevant since they are, with Enforcer, the only Thresh dependent creatures in the deck ? (I'd say yes but I'd like to know your opinions). Are Mental Notes still relevant ? (Predict could replace them ?)
Post-FS, I'm not sure about my sideboard, but I don't intend any serious changes. It'll probably be much like my Columbus list:

“Grand Prix Threshold”
By Bardo

4 Serum Visions
4 Mental Note
4 Brainstorm

4 Force of Will
4 Daze
3 Stifle

4 Swords to Plowshares
2 Engineered Explosives

4 Nimble Mongoose
4 Werebear
4 Meddling Mage
1 Mystic Enforcer

4 Flooded Strand
2 Polluted Delta
2 Windswept Heath
3 Tropical Island
3 Tundra
2 Island
1 Forest
1 Plains

Sideboard
3 Armageddon
3 Hydroblast
3 Chill
3 Loaming Shaman
2 Krosan Grip
1 Mystic Enforcer

The maindeck Enforcer can probably come out for something else. Needle and CSpells became Stifle and another EE. Though Stifle has a lot of uses everywhere, even against FS where you can drop it on Kikki-Jikki, Safekeeper/Bodyguard or Carrion Feeder. Still it seems better just to stop their turn off their combo. Honestly I'm not sure which is the way to go.

Re: The Goose. I had the same concern about Mental Note with Tarmogoyf coming into the deck post-FS, but I can't tell how nutty Note is with the Goyf. Actually, I just did.

Turn 1: Delta, pass. EOT crack Delta for Island, Mental Note dredging EE and SV (draw a card).

Turn 2: Heath, crack for Forest, 4/5 Tarmogoyf.

Those plays are easy. In some weird ways, the Goyf is alot like Werebear but ramps up quicker and is doesn't need threshold to be a serious threat. I've won most of my games in the past few weeks with the Goyf with only 4-6 cards in my graveyard. So, assuming that Goyf has a place in the deck and that it has some crazy interactions with Mental Note; then I'd say Mongoose can stay too. He's good against almost everything.

But don't take my word for it. Test it for yourself and I think you'll see what I'm talking about.