Missing Effect
Bamiyan Buddhas
Saint Sophia's Cathedral
Prambanan
Beta Giyorgis
Kazimierz
Amber Room
Crystal Palace
Nobel Foundation
Atomium
Sydney Opera House
Delta Works
Vertical Forest
Bamyan Buddhas: +1 trade route per religion present in the city. (Representing the site as a conflagration of multiple religions during the Silk Road area)
Kiev Sophia: (Maybe some generic culture value? Alternatively, because it's meant for Russia, it could decrease gold maintenance for number of cities by some appropriate value, representing its political importance at various periods of Russian history including its construction as a sort of gift by the monarch at the time.)
Prambanan: Religions that have been state religion in the past cause no unhappiness. (To represent the cause of its construction, a change in state religion from Buddhism to Hinduism, as well as the historical religious diversity of Indonesia from Buddhism to Hinduism to Islam.)
Bete Giyorgis: +1 culture to state religion buildings for every shrine in the world. (Representing the cathedral's imitation of Jerusalem.)
Kazimierz: (Generic gold effect, should be connected to non-state religions.)
Amber Room: +X Great Statesman points for each defensive pact. (The room helped forge a pact between Russia and Prussia, and Germany has a long history of building dangerous alliances in Europe: see WWI. GPP effect for Germany.)
Crystal Palace: Trade routes with cities on continents where you control territory contribute to culture. (British imperialism and trade, pretty obvious. Alternatively could do something similar with golden ages.)
Nobel Foundation: When great people are born in other civilizations, gain X% of the points required to recruit them. (Obvious)
Atomium: Every defensive pact you have generates 1 source of uranium for both members. (This might be seen as OP but the wonder will be very late game and this can represent the sharing of nuclear capabilities between European Union states as well as helping nations like Britain and Germany secure nuclear weapons in the modern age which can sometimes be difficult.)
Sydney Opera House: +1 trade route per culture level in the city. (The Sydney harbor is a very significant trade hub and the Opera House is a major tourist attraction within. If there's ever an Australian civ it could benefit from this.)
Delta Works: -1 unhappiness and unhealthiness from city size for each water tile in city radius. (To represent the transformation of sea into land which is then used to house citizens and thus makes everything less cramped. The Netherlands has one of the highest quality of living rates in the world.)
Bosco Verticale: +1 health from specialists in all cities. (For Italy.)
These are just some first thoughts of mine, thought I'd try and get something together for all of them. Totally open for criticism or discussion. Also I know we're not supposed to add more wonders but I think the St. Louis Arch should be in.