There are lots of things that could be included for London - but heres a couple of suggestions:
For Vehicles - this could be split into "historical and "modern" (or a mix of both):
Historical:
1, Feltham Tram:
http://en.wikipedia.org/wiki/Feltham_Tram (had a high capacity and central doors the trams could carry nearly 100 passengers!) Built 1931, many went to Leeds and scrapped in 1959.
2, District Line "R" Stock:
http://en.wikipedia.org/wiki/London_Underground_R_Stock Built from 1938 onwards, last ones withdrawn in 1983.
3, Metropolitan Line A60/A62 stock:
http://en.wikipedia.org/wiki/London_Underground_A60_Stock Built from 1961, last ones being withdrawn 2012.
Modern:
1, DLR stock - P86, B90 and B2007 would give a good spread of stock if as suggested, the DLR was used as a transport type:
http://en.wikipedia.org/wiki/Docklands_Light_Railway_rolling_stock
2, London Underground S stock
http://en.wikipedia.org/wiki/S_stock - 2009 onwards
3, Class 378:
http://en.wikipedia.org/wiki/Class_378 - 2008 onwards
Landmarks (but theres allot you can choose out there!):
1: Kings Cross/St Pancras, but would hope that all of the major stations are located on the map (Would be particularly nice to have a "small" terminus station) - you could even make a "Monopoly scenario" to connect all four stations on a monopoly board with a single line (Kings Cross, Fenchurch Street, Liverpool St and Marlybone Stations)
2: Big Ben/Houses of Parliament
3: One Canada Square (Canary Wharf)
But would like to see additional small buildings e.g.:
Georgian Terraced housing in the game:
http://www.flickr.com/photos/maggiejones/3250636739/
Victorian Terraced housing:
http://www.fotolibra.com/gallery/343477/victorian-terraced-housing/
Scenarios - Monopoly one as suggested, but you could also have one with Sherlock Holmes (Baker Street?), Olympics, Wembley, Millennium Dome (If represented) or one about the congestion charge? Hope its gonig to be a XL map, as London itself is huge...