I've also been working with allied NPCs and critcal allies which can add their own frustrations to proceedings.
When you introduce a buddy character to the player who must be protected or its game over, you change the nature of a first person shooter. The player has to keep an eye out for a second party. This can be used in a variety of ways that can enhance or destroy gameplay.
1: The player no longer has to worry about running out of ammo, they can fall back on their buddies firepower if they need to. Anyone who had completed HL2: Episode One achievement "The One Free Bullet" will understand that it is totally possible to survive this way if you alter your playing style.
Firefights become more dynamic if you set areas up correctly. For example, adding raised areas the NPC ally is happy to move to can give a real sense of teamwork and allow for some strategic thinking by the player.
NPC allies are a great way of controlling the flow of play. Adding gates that only the NPC can open will allow you to control when the player can move forward and can solve many gating headaches. This is seen numerous times with Alyx and the Vortigon in Episodes 1 and 2. Often it allows time to pause, deliver some story or some form of reaction to the situation. All of this makes for a more pleasing game experience if used in the right measure.
Buddy characters can also be used to make the player feel special. Alyx in HL2 repeatedly congratulates the player on being awsome as does the Vortigon. In fact, this mechanic is used so much that its a huge slap in the face when you meet Dr Magnusson at White Forest who does nothing but criticise.
Still, FPS gaming can be a lonely experience sometimes and its nice to have someone telling the player they're smart and skillful.
From a map design perspective critical allies can be a real headache. The player must never lose them, they have to be able to reach the player through the majority of the map (aside from areas designed to seperate them). Also, they should never be in a place of danger unless the player can help to defend them.
The player can get frustrated waiting for the buddy to catch up.
Ive heard from players who spent the whole of HL2 trying to ditch Alyx. "she's always there again when the next section loads!" one player moaned "I hate her". Buddy gameplay isnt for everyone but you can certainly make it far more bareable by ensuring that when the player completes a loop in the map and they've left the buddy behind somewhere, that the buddy is right there ready and waiting for them to move forward. If the buddy is half a mile away, the player is going to get peeved, trust me.
The buddy can get in the way. Always make sure your walkways etc.. are wide enough for the player and the buddy to move past each other. As anyone whose ever played Counterstrike will tell you, there's nothing more infuriating than someone camped right behind you, so that when you need to back up, you cant move. Death often follows!
Always give the NPC buddy lots of room to move around so that they can make the best of the "get out of the players way" logic they've been programmed with.
They can reduce the challenge of a map. Because NPC allies often have huge health pools it can often make the game much easier. When it comes to combat, dont be afraid to up the numbers of bad guys as the npc buddy will seriously increase the players damage potential. Playtesting should help you find the right balance.
Thoughts on vehicles
Adding drivable vehicles to your map can add great variety to the gameplay but I've found you'll need to consider some things when designing you playing spaces.
Your map areas are probably going to need to be outdoors due to the large spaces required. This means that you'll need to learn how to make displacements and get good at making them. I have found that creating template sections of maps can be very helpful as these can be used to create various layouts which the mapper can then re-arrange at will. You'll need to create a straight, a sloped straight (to go uphill or downhill) and a 90 degree turn, at minimum. A 45 degree turn section can add a bit more variety and realism. Intersperse these template sections with larger areas with more interesting gameplay.
How do you make sure the player doesn't lose the car?
This is a bit of a tricky one. Where a car can go... people can walk... so how do you make sure that the player brings the car with them into a new section of the map where they are going to need it?
How do you make sure they dont end up stranded with no transport?
Well I've come up with several potential solutions...
1: add a jump that only the speeding car can clear
2: add a wall/gate that only the player can break through in the car
3: add an electrified floor
(not very realistic...)
4: add more cars. Add a spare car in each new section of the game.
5: build a route for the player to go back and get the car on foot...
I may employ one or more of these methods in Deep Down... we shall see!