First off, why not just do what previous editions did and use the two-handed longsword as a katana (yes; yes; I know, but it is effectively the bastard sword in 3.x. Let's not split hairs here.)
Some of your abilities feel a bit uninspired.
May I?
School of Training
Samurai often spend the majority of their lives devoted to a particular school and style of fighting. Pick one school to follow, and at 3d, 7th, 10th, and 15th level, gain the ability associated with your chosen school. These schools are laid out below. At 18th level, however, all Samurai gain the "Ancestor's Terror" ability.
Katori Shinto Ryo
All around discipline. Focuses on techniques both with and without the blade. Other weapons beyond the katana are also practiced.
Servant's Gaurd (3rd Level): When wielding a two-handed weapon, you may choose to use your reaction to give an ally within arms-reach (read: 5ft) of you some protection from that attack. (This functions as "Protection" fighting style, except with a two-handed sword instead of the shield.)
Honor's Call (7th Level): As a bonus action, choose an opponent within earshot. They must make a wisdom save equal to 8+Prof Bonus+Cha Modifier. If you are successful, they gain disadvantage against any target other than you for 1 minute. This ability can be used Cha mod (minimum 1) times per long rest.
Harder than Steel (10th Level): You may use your bonus action and attempt to destroy a target's armor instead of dealing damage. Make a strength check against your target's AC, and if successful, you halve the armor's bonus over 10 (rounded up). For instance, if a target is wearing Half-Plate (which provides a flat armor bonus of 15) and you sucessfully use this ability, the armor is partially sundered, only providing 13 armor. This ability can only be activated successfully once per individual.
To the Bitter End (15th Level): When you are reduced to 0 or fewer hit points, you may immediately make some number of melee attacks before falling unconcious. You can choose to take several attacks before falling unconcious, but for each attack beyond the first two (up to your max in an attack action), you mark down a failed death saving throw. (i.e. if you immediately take four attacks as part of this ability, you fail two death saving throws.)
Megi Ryu
A particular school of Iajutsu in which one draws and strikes with the blade in one fluid motion. The strengths of this school are short, deliberate movements with counter-attacks. Dueling against lone opponents is the trademark of this defensive style.
The Bird's Gamble (3rd Level): As a bonus action on your turn, you may choose to give one attacker disadvantage on all attack rolls until the end of your next turn. However, other attackers gain advantage against you.
Between the Waterdrops (7th Level): When you make a successful dexterity saving throw, if saving would still deal half damage, you take no damage instead.
The Yielding Willow (10th Level): You may use your reaction to cause an attack that misses you to hit a different target you designate within range of the attack. The roll must still meet or beat the new target's AC (as per usual).
Still Blossoms (15th Level): Once per long rest you may choose to act first in initiative and no enemy may gain advantage from any source against you until the end of your next turn. If you are caught unaware and use this ability (such as in a surprise round), you automatically dodge until the start of your next turn. You may also now take the dodge action as a bonus action on your turn Dex Mod times (minimum 1) per long rest.
Niten Icchi Ryu
Founded by a now-famous master swoardsman, this school incorporates a two-weapon technique.
Extension of Body (3rd Level): The Katana is now a light weapon for the purposes of two-weapon fighting. Additionally, when not wearing heavy armor, unencumbered, and using a weapon in each hand, gain +1 to defense.
Ring of Steel (7th Level): Gain an additional +1 to armor when using two-weapon fighting. Additionally, you may use your reaction to give this (now total of +2) armor to a nearby ally (within 5ft). If this armor increase would successfully block the attack, then the ally you've defended may immediately use their reaction to make a melee weapon attack. You must be using a weapon in each hand in order to use this ability.
Lash Like Flames (10th Level): If an attacker misses you, you may immediately choose to take your reaction and make a melee attack with both weapons (as though you made an attack with each). You must be using a weapon in each hand in order to use this ability.
Embrace the Void (15th Level): You gain proficiency in Wisdom. Furthermore, when you are the subject of a mind-affecting spell, you have advantage on all Wisdom, Charisma, and Intelligence saving throws.
And then we'll finish off with your level 18 ability, because I thought it was pretty cool.
All the different schools get this.