Don't know if this helps, but in my homebrew game, we've defined a few different types of reach:
Reach
Reach weapons are used to strike opponents 10, 15 or even 20 feet away, depending on the weapon (see the weapon's description), but cannot be used against an adjacent foe, nor does it threaten adjacent squares.
Adaptive Reach
A weapon which offers different grips, allowing it to be made into a reach weapon as a swift action, and returned to non-reach as another swift action. While using it as a reach weapon, it does not threaten adjacent squares and may not be used to make attacks against targets in adjacent squares, until returned to its non-reach grip. While wielded as a non-reach weapon, it may only attack adjacent squares, even if the weapon is large-sized or has more than 10 feet of reach in its reach-weapon grip.
Inclusive Reach
An inclusive reach weapon has the reach property but can also attack and threaten adjacent squares.
We've also got a couple of weapon properties related to reach:
Unwieldy
An unwieldy weapon requires a great deal of space to properly use. If any adjacent square is considered blocked you cannot use the weapon, and if any square within ten feet of the wielder is considered blocked you have -4 to hit. A space is considered blocked if there is a permanent object which takes up enough of the square that movement is not allowed into it. Typically this is a wall or pillar, but statues or other map features may constitute a blocked space. Creatures or low terrain elements like tables or stairs do not cause a space to be treated as blocked.
Cumbersome
This weapon requires space to maneuver freely. If any adjacent space is considered blocked, you receive a -4 to hit. A space is considered blocked if there is a permanent object which takes up enough of the square that movement is not allowed into it. Typically this is a wall or pillar, but statues or other map features may constitute a blocked space. Creatures or low terrain elements like tables or stairs do not cause a space to be treated as blocked.
And there's a couple others which aren't specific to reach but come up often (like with the whip):
Non-Threatening
Weapons with this property do not threaten squares, and thus cannot be used to make attacks of opportunity. Additionally, wielders of such weapons cannot assist with flanks.
Provocative
A provocative weapon provokes attacks of opportunity from foes which threaten the wielder when the weapon is used to make an attack.
Using these weapon properties, we were able to define a number of weapons which were relatively balanced within the game. Most of the truly good reach weapons require an exotic weapon proficiency, however, as reach is very powerful in Pathfinder.
To your specific question, does the weapon have to Unarmed Strike, or could it be new weapon you define? Unarmed Strike has so many strings attached to other things, I've found it very difficult to tinker with.
Good luck!
Nuada
