Greetings!
I'll start off saying I'm on my 2nd play through, so I'm familiar with re-spec options beyond Act 1.
1) Re-spec Process. The need to go to Lady Vengeance and click a magic mirror that has no backstory vice just having a menu button is annoying. There aren't even practice targets near the mirror in order to test out your new builds. You can already teleport all over the map, I don't see much of an "immersion" issue with having a menu button to re-spec. Implementing this would allow an Act 1 re-spec option without any additional narrative. If a menu button option is a step to far, then how about some kind of magic artifact you keep in your bag and can click to re-spec. Grinding through Act 1 with a spec you don't like is one of the more frustrating things about this game.
**If the below suggestion already exists then please disregard, but I haven't run into this feature during my first play-through**
2) Practice Targets. There should be an area in the main town of each Act that has practice dummies. This creates a nice clean environment for folks to test builds and theorycraft to their hearts content. This is a standard feature in many RPG type games I've played over the years that is widely enjoyed by the player base.