The world of Monster Hunter is not currently very involved with anything close to magic (not counting the Chameleos weapons). However, there are those special moves the monsters we fight use against us... Wouldn't it be interesting if we could use a condensed version of Rathian's fire bomb or Tigrex's roar against another monster?
To introduce a weapon like this, I thought immediately of how the Switch Axe was introduced in MHTri. Relatively early in the game the Artisan blessed us with a weapon so advanced and fun to play we couldn't help but drop whatever weapon we were playing and pick it up. In whatever installment Capcom decides is ready for the introduction of magic, the smith or another NPC could introduce the Mystic Spear similarly. Along with the weapon, the ability to manufacture spells from monster parts and upgrade them.
This is Monster Hunter, however, and simply standing on the sidelines hurling spells like a madman would not be fun or give a sense of accomplishment. The Mystic Spear balances its powerful spells by making the player earn them. Hitting with the two handed Spear (which can have the design of any polearm such as a halberd or naginata) charges up the spells the player has in their gunner pouch similar to how Wystones charged over time in MH4U. When they are ready for use the player selects up to three of the same type (more on types later) to dole out at once and presses the right trigger, unleashing destruction upon any unwieldy beast. Another counter presumably under the sharpness indicator would fill up the more one uses spells and when full, adds the ability to charge attacks for the Spear similar to the Charge Blade's sword. Charged attacks charge spells faster, which in turn fill the Spear. The more powerful spells take longer to charge, but reward the player with over the top chaos indicative of classic Monster Hunter. The Spear, contrary to the name, mainly uses swipes and swings that deal cutting damage. Some moves would indeed jab, causing piercing damage, but would be hidden at the end of the Spear's many combos. It is too large to dodgeroll with, so evading would cause the player to sidestep omnidirectionally similar to the Dual Sword's arch demon mode sidesteps.
As for armor and skills, Mystic Spear users can equip both gunner and blademaster armor, but can not mix them together (they can either equip all blademaster or all gunner armor). Gunner skills will affect spell use in many ways while blademaster skills affect the Spear. Armor type dictates whether the player will prioritize the Spear or the spells.
Spell types: spells will be classified by elements and usage. Normal, Fire, Thunder, Water, Ice, and Dragon. Normal type spells, forged from monsters like Tigrex, affect the environment. As for usage, there is player/group buffs, shields/counter attacks, AoE/splash, projectile, and close range.