I have a game in which the first-person view is useless due to low FoV.

Would it be hard to create a directx wrapper that increases the FoV of the game?

I have a game in which the first-person view is useless due to low FoV.

Would it be hard to create a directx wrapper that increases the FoV of the game?

FOV isn't dependent on the graphics library since it has nothing to do with the GPU because its a simple mathematical calculation used in a transformation matrix. Your best bet would probably be to use a hex editor like cheat engine or ollydbg and isolate the location of the FOV variable and change its value (provided its not hard coded in the game.)

This is assuming that this is an already existing game that you don't have the source code for, if it isn't I guess I completely misread the question.

That's true, cheat engine might be a better option.

I'm almost certain that the game uses Unity3D 4.7 or newer, so I'm assuming it must have a FoV variable somewhere.

Yes if you could intercept the D3D calls, find the projection data, and replace it with your own, then you could do this.
...
But, most games also do frustum culling, where they don't tell D3D about objects that are outside the FOV, so there would be a lot of "popping" and glitches within your extended FOV region. You'd be better attacking the game code directly, as mentioned above.

Also, if the game has anti-cheat or anti-piracy measures, you might trigger them...

It's a non-competitive social game, but it's always-online and seems to be very hardcore on anti-piracy.

I read their ToS and it only says that I can't modify the executable or the client/server communication transmissions, so technically editing runtime client memory should be fine. I think I'll try to ask one of it's developers to avoid getting my account banned over a broken first-person view.