The reason trainer works as well as it does is because it promotes limited use. When you get on trainer, you have to make a new account, click every rare 1 by 1, and make a new deck. Every time. It does get annoying if you do it too often. But that's the point.
If you implement PvP, deck saving, and features like that, Trainer just because Elements on God Mode. There would be no reason to work on the game, no reason to grind at all. You could have a fully upgraded version of every card in the game, and it would take a couple of minutes if you can click fast enough.
Maybe, just maybe, the option of seeing your opponent's hand should be there (as a box to the side, some sort of on/off)
I'm all for being allowed to modify the current match you're in. Make trainer more like a sandbox mode. Not only can you test decks, you can go 'What if this happened?' This let's you test your card situations, and even lets dedicated players do specific bug testing. It would be difficult, but options to choose your cards you get, set life/quantum totals to specific values, and things like this, would be useful for situational testing.
Some people might see this as an unfair advantage, but it's the trainer. Players who use this for anything other than testing really aren't getting anything out of it.
My only suggestion (I really don't think it deserves it's own thread) is to have a way to distinguish trainer from regular in screenshots. For example, if you look at the main menu screen, where it says 'Main Menu,' maybe during matches it would say 'Trainer'
It could come in handy at later times, when we want to do events where the players would need to be playing their own decks.