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Here is a project of an enormous high-rise between Fuzhou Road 福州路 and Yuanfang Lane 元芳弄, which would have called for the demolition of the former Great Northern Telegraph from 1908 (7 on the Bund 外滩7号), the Shanghai-Nanking Railway Administration (Yuanfang Lane 元芳弄) and the China Merchants Bank (9 on the Bund 外滩9号). The latter banking group were, in fact, pushing for this redevelopment plan, but it did not materialize and we still have the old buildings.
Image: Building Shanghai
What we have:

Image: bigpixel.cn
[See the rest of the projects...]
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Image: Shanghai Sunday Times, Dec 1931
We can only regret this plan did not go forward, although Minutti got to build the Messageries Maritimes several years later (on the far right of the modern image):

Image: bigpixel.cn
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Image: The Builder 建筑月刊, 1935
What we have instead:

Image: mapio.net
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Image: North-China Sunday Times, Dec 1931
Let's enjoy it from another angle:

Image: Shanghai Sunday Times, Dec 1931
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Image: avezink
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Image: The Builder 建筑月刊, 1934
Hajek's skyscraper would have at least been funny, but what emerged there in the new century is plain boring:

Image: bigpixel.cn
If I think of other unrealized Bund buildings I'll add them here.
UPDATE:
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I thought of another unrealised building, planned for Whangpoo (Huangpu) Road – the American Consulate. In fact, there were at least two versions. One was proposed by R. A. Curry in 1926:
Another one was proposed in 1935 (I cannot find the hi-res drawing in my files, so I'm using one from wikipedia). This massive symmetry resembles some Hong Kong projects of the era, and it is also somewhat awkward:

In the meantime, in another part of town, here is what the Credit Foncier d'Extreme Orient wanted to put on Avenue Joffre (Middle Huaihai Road 淮海中路) in 1931, between today's South Xiangyang Road and Fumin Road:

Instead, that block has K-Wah Centre:

Image: MatchOffice
There were many more ambitious and unrealized projects in the French Concession. Perhaps I should make a separate entry about them.