1.d4 e6 2.c4 Bb4+

This rare sideline is pretty annoying. After 1.d4 e6 2.c4 Bb4+, 3.Nc3?! doesn't seem to be something worth playing, so it seems 3.Bd2 is necessary. However, I don't know what I'm aiming for in the positions after 3...Bxd2+. How can I play for a win in this position with the bishops traded as white?

3.Nc3 is playable but it is an improved Hubner Variation of the Nimzo for black, so I don't know why I'd want to play it as white, hence why I gave it ?! as it seems to be a bit of a mistake.

I might consider 3.Nd2 though, that seems to be interesting. 3.Bd2 is supposed to be the best move according to theory though.

I don't know why you feel 3. Nc3 is so bad. It's a transposition to the Nimzo-Indian which is still a frequently played opening. If it were a dubious move the Nimzo-Indian would disappear from tournament play because White simply would not allow it. What you seem to be suggesting is a transposition to the Bogo-Indian which I think offers White fewer chances for a win than the Nimzo-Indian. If you're fearful of the Nimzo-Indian however then 3. Bd2 is the most solid move but if you're playing hard for a win then 3.Nd2 is probably best hoping to force Black to trade Bishop for Knight and thus gaining the Bishop pair. 

You will never find a forced win for white in chess! Even the most powerful computers cant find a forced win. Statistically, more than 1 billion games were played by stockfish  and there is no forced winning line for white. White player's initial advantage is so small to forced win while there is a big margin of draw in chess. If there is a forced winning line in chess, everyone will play that line and  this game will disappear. 

This is all true of course but when players ask about what opening or variation offers White the best winning chances they are thinking in terms of human vs. human competition and what might give the human playing Black the most problems to think about.

2....Bb4ch is one of my favorite lines as black. It is recommended in Eingorn's " 1...e6 solid rep". It is a bit much marking 3Nc3 as dubious as it has been played by very strong players like Alekhine v Keres Avro '38.  It does have the advantage for black of avoiding the classical Qc2 Nimzo after 3Nc3 c5. Also going into English defense lines with 3Nc3 b6. The position which you are calling an improved Hubner, is really a Hubner like position were black is avoiding Nf6, and theoretical positions. It is not clear if it is better for black than the positions with Nf6.

3Nd2 Nf6 going into Bogo-Indian lines is possible. Black can also try 3...c5.

It's not a transposition to the Nimzo, black doesn't have to play Nf6. If it transposed to the Nimzo I would definitely play 3.Nc3

For example after 3.Nc3 b6 the game has transposed to a good version of the English Defense for black.

Nimzo Indian is my black choice against 1. d4

Very notorious drawish opening line against 1.d4

( the same as Berlin against 1.e4).

For black player, it is very easy to equalize in opening. That is all you need to do as black in the opening. I fight for some winning chance in endgame though.

Are you talking to me?

Oh, well. I just play the same opening vs 1200, vs 2000 or vs 3500 Stockfish that I practice at home.

Of course whatever opening you use( unless you exchange too many pieces) the weaker player will likely choose weaker moves later and self destruct, and the stronger player will play stronger moves and outplay you.

Your evaluations are nearly all wrong.  3Bd2 BxB 4QxB Nf6 5Nc3 d5 is a Queens Ganbit declined similar to the Lasker variation, if white plays e3 etc. Otherwise the closed catalan positions with g3 are causing FIDE 2700 level players problems showing any advantage. Carlsen has played the dutch position after 4...f5. A common theme is exchanging the good bishop isn't bad for black. For example in the closed catalan after g3, black often finds his remaining bad bishop is more effective working from Ba6, after b6, than white's good bishop on g2. Also using two bishops, a d4 player favorite, is immediately not possible. In general what stays on is more important in evaluating a position, than what is exchanged off.

                    A lot of the other variations are tranposing to known Bogo-Indian ones, were your evaluation is optimistic for white. In your main line, instead of 4...Nf6 another known Bogo line, black can try 4...Nc6 then d6, and e5 attacking d4 early, trying to exploit delaying Nf6 further.

Nc3 and 0-0-0 and f3 with a crushing kingside attack to come

That's your main problem, you are evaluating white's positions being very favorable due to one factor, black exchanging his good bishop. In http://www.chessgames.com/perl/chessgame?gid=1528284 Carlsen eventually plays a stonewall without the black squared bishop, and wins. Its not the not the only time Carlsen and other strong players have done this.

For OP, According to Alpha Zero, his choice is Bd2.

Check video at 1:20 min

https://m.youtube.com/watch?v=MAbUiAaqjPs

That's not the same position at all, but a much played Queens Indian line.