Brickipedia (A.K.A. lego.wikia.com) list unreleased sets and themes in the form of an article and category of articles. It does mention CYBOTS and Europa in addition to Seatron, although there is no guarantee that the list is complete.
Contained action figures using both System and the first TECHNIC
ball joint parts. Several prototype models ...
In this forum, I found a reply from the Lego Group in response to an email asking a similar question but related to the Mars Mission games:
Thank you for contacting us and for your interest in LEGO® brand products.
Unfortunately, because The LEGO Mars Mission line you inquired about has been discontinued from our current product line, we do not intend to ...
LEGO xtra is a theme of sets that focus on adding play value to your existing sets by providing accessories or play areas.
For example, LEGO xtra Traffic Lights adds 4 buildable traffic lights and a few associated accessories to your city streets:
These sets sometimes feature playmats or baseplates as well, such as the Sea Playmat set:
These sets are ...
What about the 75930 Indoraptor Rampage for an almost timeless mansion? Without the raptors it is not too expensive.
The 10228 Haunted house could provide a good basis for a rebrick of a nice mansion on the hill just outside of your city. However, this one is ALWAYS expensive, as you probably know.
For many nice little ideas for everyday items and a MOC ...
In the blog of Tom Aplhin he mentions regarding Microscale
Steve Oakes, a long-time LEGO builder in Seattle area started a tradition 4 years ago to re-create the BrickCon Public Convention at a smaller scale. His display is called “Micro BrickCon”
(Edit: The conversation between Tom and myself was in a loud room with lots of activity. He is correct about ...
I can't believe that nobody else else has posted the most important real company that appears within the LEGO City theme... the LEGO company itself!
There is a good list of promotional sets on Brickipedia, that includes the set theme. Many of the town / city sets include logos or brand names of real-world companies.
Other sets that include real-world ...
From 2002-05, LEGO sold Spider-Man themed sets under license from Marvel Entertainment in conjunction with the release of the first two Spider-Man movies. These are commonly known as the Spider-Man 1 and Spider-Man 2 themes. This license was apparently only for Spider-Man and characters in the films and not for other Marvel superheroes.
In October 2004, ...
Here are some additional non-fictional brands and their set(s) under the Town theme:
Chocomel (Nutricia), Holland
ANWB (Breakdown service), Holland
Roadside Recovery Van and Tow Truck
Touring Club Schweiz
Air Canada Jet Plane
As I am not a LEGO employee I can only guess, but I would say the LEGO Service line got replaced with LEGO Pick-a-Brick (and LEGO Service’s Bricks & Pieces) as a way for customers to get their hands on quantities of specific pieces. The LEGO Store Pick-a-Brick wall serves a similar purpose.
Lego also collaborated with several airline companies to release a standard plane but with stickers denoting the airline name. I remember excitedly purchasing them back in the day. They were only available through their in-flight shopping if I recall correctly. I have two. One is El-Al and the other one is for a Japanese carrier (need to check either ANA or ...
I wouldn't consider "hospital" as fictional.
At first, LEGO did seem to use the Red Cross logo, but they have since then had to stop (presumably because of the Red Cross being very protective about their logo). Note that the logo they have been using since isn't fictional either.
12v era trains provided stickers for a lot of different train companies, I'm ...
Real brands include:
Mostly it's the three early Maersk Trucks - 1552, 1831 and 1651 that are usually listed under the Town theme.
Toys R Us also appeared on one City themed truck
Outside of the Town/City themes but still minifig scale you then have:
Maersk Train - under Trains.
SNCF - In it's old orange livery is the basis for the Horizon Express train ...
Both the Master Builder Series and UCS will continue. Here's the official distinction from the LEGO Star Wars team:
The Master Builder Series models are large playsets and beside being complex builds they are characterized by having many play features and functions, interior details as well as a range of minifigures. Ultimate Collectors series will remain ...
Start by looking for some rarer pieces. Not regular 4x2 bricks, but maybe a slide or a odd-shaped wall piece. Under each brick will be a four or 5 digit number. Go to BrickSet.com and search for that number, hopefully it will find the piece you are looking at and list colours that piece is available in and the sets it comes in (e.g. http://brickset.com/parts/...
While there is no explicit "house" tag, I would recommend browsing in Brickset's City theme: http://brickset.com/sets/theme-City
You can refine the search based on year (include only recent, therefore easy to buy sets), tags (building, shop, ...) and subtheme (airport, construction, police, ...).