First, you have to decide what equipment you are going to wear.  Taking everything would weight too much, so you have to limit yourself.  Should I bring a camera and a safecracking kit to gather more evidence or should I get grenades to take out the guards more easily?

Then, you have to search the house.  You hide from the guards to prevent from sounding the alarm. 

The mouse cursor will change into a target icon when it is over a guard and clicking on him will shoot a bullet.  The more precise the shot, the more damage you will do.  Precision depends on distance and the time you took to aim before shooting.  The smaller the circle in the target icon, the more precise is your aim.

If you take one guard down without sounding the alarm, you can take his clothes to disguise yourself.  With a disguise, you can be seen by other guards in the same room, as long as they don't see you face to face.

You can search and take photos for clues and evidences.  You can also place some bugs to monitor movements in those rooms even if you are far from there.  Searching in a safe usually gives you better evidences or equipment.  It might even give you the master plan or enough evidences to make the criminal agent turn on his organization and work as a double agent for you!

Done:

• Graphics for an office type of room.
• Graphics for the guards.
• Generate a room making sure there is a path between every doors, i.e. the furniture does not completely block the access to a door.
• Miniature of the house:  Unvisited rooms are not shown, as are guards in rooms not bugged or too far away.  Guards in the same room as you show up only if the room is bugged.
• Agent UI to move, open/close door, search, safecracking, take photos, plant a bug, stand/crouch, aim and fire pistol.
• Alarm when you are discovered.
• Guard AI to move around the house, fire pistol and respond to an alarm.
• Line of sight: Guards out of sight will not show up.
• Mouse cursors for walk, stand/crouch or aim.  You can also use the spacebar or 5 on the keypad to stand/crouch.
• Health for both you and the guards.  The amount of damage done by a bullet depends on the time taken to aim.
• Sound effects.

Left to do:

• Graphics for other types of room.
• Grenades, booby traps and remote control.
• Jump
• The car junkyard "room" used when criminals drive you off the road because you raised the alert level too high in the city.
• Music
• Inventory system: Beside the items you could pick in the original game, I plan to add a police helmet (extra hit point), smoke grenade, dynamite stick (opens a safe, but start the alarm), test tube with acid (opens a safe without starting the alarm, but might break if you get hit while carrying it).
• Disguises and search corpses.
• Computer access and password.

Ideas for improvements:

• Make the suspects move inside the house, maybe with bodyguards needing more than one hit to shot down.
• Suspects really go to meetings and to the infraction sites.  So, if you break in when the suspect is away, you can hide somewhere waiting for him to come back.  If it is time for the suspect to go to a meeting or do some infraction, he might get out of the house and you can follow him.
• Some kind of puzzle when you try to open wall or floor safes.  You have to be careful to keep an eye on the breaking into building window to make sure the guard do not surprise you while you do this.
• More logical placement for furniture than random.  Random has the advantage of making combat inside a room more different than if it is more logical, but it would feel less weird.
• Multi-level houses with stairs and/or elevators to change level.  To prevent having to spend hours on a single house, there would be less rooms per level.  The sound map also shows the room just over and under your current location so you know if you can safely climb up or down stairs.
• Surveillance cameras and you might have to disconnect them to go undetected.  Disconnecting them could be done in the electronics screen.
• Starting outside: instead of selecting a door and starting inside, you start outside and have to move to the door (or maybe a window?).  There would be very little cover outside, so it would not be a good place to stay when there is an alert.  When you take down guards, reinforcements would come in part from the outside.