I really love the look of the new (well actually old, but never seen in a movie), Sith Lightsaber
That lightsaber is the silliest things in Star Wars since the idea of Midichlorians (for the record, in my headcanon they do not exist and Qui-Gon just did a normal medical blood test that had nothing to do with the force).
The way a guard functions, mechanically, is that it prevents the opponent's blade to slide along your own blade until it reaches, touches and injures your hand or arm. Instead it hits the cross guard and is stopped there. Mechanically, this forces the opponent's blade into the very notch where guard and blade meet; this part was therefore typically reinforced and not sharpened. Now, in that lightsaber with its cross-guard, this very notch is the most vulnerable part, because it consist of solid matter and not a laser-plasma-force-field-whatever blade. The enemy's blade would cut right through it and your saber in half, and then your arm.
And before anyone interjects that the SW lore has certain expensive materials that can withstand a lightsaber blade (I am not sure whether that is even official canon) - why then not construct the guard itself out of this material, too? That would remove a lot of the dangers that arise from a cross guard made like a lightsaber-blade. Don't forget that it is one of the centerpieces of the lightsaber that it is so dangerous that even with lots of training, to an ordinary person without force-sensitivity it would pose as great a danger as to themselves an opponent's weapon.
But then, it is likely that a cross guard would be unnecessary anyway. In most lightsaber fights it can be seen that two blades that connect never slide off one another. Instead, when forced against aech other, they can even somehow interlock.
Last but not least: it doesn't even look good with he cross guard in the first place. Lightsabers are meant to be elegant, sleek things. Therefore I believe the only reason why the thing exists is because the maker did it out of pure vanity and without any practical reason whatsoever, maybe saw an ancient steel sword in a museum and thought that would look cool as a lightsaber. Even a Sith may sometimes err and have bad taste.
