In short, I think it would be right if a shield user could only block attacks from the front. (AFAIK right now shields give a chance to block attacks from any direction)
But in DF there are no "fronts" and if there were you couldn't tell without advanced graphics where <creature> is looking, and it would complicate things because you would have to turn in place in adventurer mode so the idea is like this:
1) A dwarf (replace dwarf with any other creature) cant block a surprise attack.
2) A dwarf can turn in place pretty fast. Fast enough to try to block one attack from any direction, if he's aware of it.
3) A dwarf can try to block several attacks if they are coming from one set of directions, depending on the shield (and skill - that would be nice). For example a buckler and novice shield skill result in covering 1 directions out of 8, but a larger shield or better skill would cover 2. Like this (0 are not covered directions, # covered, d - dwarf):
buckler
0#0
0d0
000
shield
0##
0d0
000
A legendary shield user
###
#d#
#0#
A dwarf with a shield and novice skill is surrounded by carp. A carpy situation it is.
aaa
ad0
0a0
Carp 1 attacks, dwarf defends and now can only try to block attacks from carps marked by #.
#1#
ad0
0a0
Carp 2 attacks and the dwarf tries to block. Regardless of whether it was blocked or not, the dwarf can't block attacks from other directions, until he gets another turn and his blocking position is reset.
21a
ad0
0a0
It means that until the blocking position is reset, the dwarf can try to block any number of attacks from directions 1 and 2 but not from others. He's fully open to 3 carpy attacks.
Doesn't it suck? Well of course it does! To compensate, the chance to successfully block an attack should be increased. Also, the same rule works for ranged attacks too.
First of all it should weaken ranged (well and melee too) attacks in adventurer mode against shield users. One person cant attack from several directions. To make things better for those without shields, a parrying can be introduced in a form of x/8 directional chance to block using the weapon skill instead of shield skill. (it is sensible to make x a worse modifier than one of a buckler, for example 0 at lower skill levels, which means no chance)
Also it will make attacks from several directions against player more effective in adventurer mode. On the other hand it would mean better chances to get close to a single shooter (nerfing archery again)
In fortress mode, it would mean that surrounding is more effective than attacking from one side. It should significantly weaken ranged attacks from one direction.
The idea is still raw and unrefined I just thought about it and wrote right away.
edit: the first attack may or may not indicate the "center" of covered area. If it does, even a legendary shield user can't block attacks from 2 opposite directions.