One problem with testing Standard decks against Legacy is that you have no idea what the Standard Metagame is, and you get no good ideas for fresh Standard Archetypes.
I completely agree. Hence the push on the player base’s side of things to get D3/Oktagon to get a standard version of TG up and running quite some time now.
To be fair, Across Ixalan is “sorta/kinda” it.
But realistically, it’s hard to call it that when it’s only the realm of free for Platinum only, and also color limited on a cycle.
Don’t worry! Standard version of Training Ground is already in the works 
I primarily use Across Ixalan as a TG proxy, though it has a number of board effects that skew testing. It’s a good way to see other Standard decks, though an increasing number of people seem to just play Bolas Omniscience decks three days a week. =)
If you want to test your deck against a vicious PW environment try the Heroic Encounters - Any of the individual PW’s Chapters in Story Mode. And you get 250 MR instead of 100. 
