Many architects and urban planners have designed conceptual city plans, most of which are meant to be "Utopian". Most were never actually meant to be built, but the architects of these plans hoped that their ideas could one day be implemented into existing cities.
Ville Contemporaine by Le Corbusier
http://www.archdaily.com/411878/ad-c...e-le-corbusier
Ville Radieuse by Le Corbusier
http://www.willofmemory.com/?page_id=299
Cite Industrielle by Tony Garnier
https://senacatal.wordpress.com/2016...dustrial-city/
Uxcester Garden City by David Rudlin, a modern version of Ebenezer Howard's Garden City plan
http://urbed.coop/wolfson-economic-prize
World Capital by Ernest Hebrard and Hendrik Christian Andersen
http://digitalarchives.mundaneum.org...QQ_clnh7?hl=nl
Welthauptstadt Germania, plan for Berlin in Nazi Germany
http://utopicus2013.blogspot.ca/2013...-germania.html
New Jerusalem by Cyrus Teed, near present day Estero, Florida
https://www.floridamemory.com/items/show/258022
Futurama at 1939 World's Fair by Norman Bel Geddes
http://www.oldcarbrochures.com/stati...turama-18.html
Democracity at 1939 World's Fair by Henry Dreyfuss
http://www.annaadahl.com/work_in_progress/democracity/
Metropolis of Tomorrow by Hugh Ferriss
http://www.graphicine.com/the-metrop...-hugh-ferriss/
Broadacre City by Frank Lloyd Wright
https://www.pinterest.com/pin/529384131172281183/
City Beautiful Plan for Chicago by Daniel Burnham
https://www.recivilization.net/Ameri...ybeautiful.php
City Beautiful Plan for San Francisco by Daniel Burnham
http://www.trampsofsanfrancisco.com/...-park-part-vi/
City Beautiful Plan for Manila by Daniel Burnham
http://www.gmanetwork.com/news/lifes...chitect/story/
Roblin City by William Bruce
http://www.mhs.mb.ca/docs/mb_history...blincity.shtml
Zion City by John Alexander Dowie
http://urbanplanning.library.cornell...S/schermer.htm
Also, not really a Conceptual City, but Fake Omaha is a project by Neil Greenberg that is meant for skyscraper and urban planning enthusiasts such as ourselves. Its an open platform for your own ideas, and one of my favourite projects out there. The "explore" link is broken, but the description of the project and map of the fictional city are available to explore.
http://www.fakeomaha.org/
If anybody knows of any other conceptual city or unbuilt city plans, please post them!