I believe my answer would lead to a bit bias, but I will give my reasoning on why those regiments should be in-game.
- Iron Brigade - 2nd Wisconsin Infantry - Heh, I said it. A regiment with a unique history, uniforms and weaponry compared to most of the Union Army. They carry long frock coats, black hats and the powerful Lorentz Rifle as a main arm for the 2nd Wisconsin, making them a death machine on the battlefield, as were the men of the Iron Brigade as a whole.
- Eastern Iron Brigade - 14th New York State Militia - The Red Legged Devils, a wonderful unit that used the Enfield Rifle, had wonderful uniforms and were significant in a number of battles.
- 42nd Pennsylvania "Bucktails" - One of the most unique units on the Union side, entirely taking in skirmishing tactics as the base of their regimental formations and training, having accurate rifles and being equipped with fancy bucktails on their caps, they are one of the most unique.
- 39th New York "Garibaldi Guards"- An extremely unique unit that is entirely made up of European settlers, each company having a different nationality. They carried a long frock coat and carried the abbreviation symbol of their regiment on their hat.
- Irish Brigade - Any of the Regiments - The Irish regiments were a unique Union force that had their own songs, style of fighting and origin ofcourse, topping their hats with a symbol of home and nature.
- 52nd New York "Siegel's Rifles" - A good german union regiment with a great amount of history.
- 2nd Rhode Island - A fancy unit with a fancy history, unique uniform containing grey trousers instead of standart union light blue.
- Perhaps a regiment I am not listing, but some that carried full-grey when it went into action during the war.
- Stonewall Brigade- Virginia Military Institute Cadets - A wonderful unit that requires no explanation why it needs to be added.
- 1st Texas Infantry - Wonderful regiment, wonderful texans, wonderful uniforms. Add it.
- Generally units that started using rifles, instead of smoothbores for some balancing.