The complete list of 2D options I know of would be:
- Complete Top Down
- Birds View
- Side scroller with and without depth
- Isometric
In top down you see everything exactly from the top. Then you only need one graphic for your mobs and you can rotate it. You can also have 360° degrees of freedom in aiming and or movement. Unfortunately usually does not look very nice.
Birds view is kind of like top down but you also see one side of all objects. Like the first Command & Conquer. Looks better, but you need images for all mobs in 8 direction. Its also harder to have more than 8 directions, except when you use true 3D, because you just do not want to make so many different views of your mobs.
Side scroller without depth is your usual classic jump and run. Depending on your gameplay your movement is pretty limited, usually sideways with some vertical in between like ladders, jumping and stuff. In the easy case you only need 2 views for each mob, to the left and to the right. If you want free aiming and the character to reflect that like aiming the weapon in that direction, you need more views for the aiming directions.
Side scroller with depth, like Golden Axe, adds some depth into the side scrolling where you can move more towards foreground and background. Still you usually only have a left and right view of each mob. Free aiming gets kind of complicated, because your crosshair does not reflect the depth of the scene and you cannot easily determine if the player aims high in the foreground or low in the background.
Isometric is sometimes called 2.5d I think and its the most 3D like optics you get, but also the most complex to do. You need at least 8 directions for each mob graphic and you may have multiple layers in the level, need to determine when you want to show the roof of a house and when to show the inside and lots of stuff.
As for aiming for certain parts, with perspective it gets complex because whenever you have theoretical 3D with a fixed cam and your crosshair only is a 2D coordinate on screen, you do not have a unique 3D coordinate where the crosshair points at, rather its a line through the scene. You can do things like lock the height of the aim, lock on the nearest enemy or do some kind of best guess.
On top down, you can either have a dedicated head target zone within the body target zone. That would also apply to all kinds of monsters and creatures where you maybe want different vulnerable areas. You could also make it just random luck by making 10% of body hits a critical, but well thats random.
It depends on the prespective on what you can aim at. There will always be some kind of overlapping. Top view will overlap head with body and body with legs. Side view overlaps arms with body. Isometric Is complicated.
If the aiming on zones is important for your gameplay, I would go with a clean view either side or top view. Side view gives the best exposure for head zones. Top view allows to define all sorts of critical zones on the borders of the mob target zones. Just overlap critical hit zone circles with a bigger body circle and check for a hit against the body and against critical zones, the critical zone wins. The optics may be less pleasing, but there are at least very beatiful side scrollers out there, I am not sure about top downs. This also limits your graphical work as you do not need too many perspectives of each mob.