Huckleberry but apparently they are really difficult to farm.

Thin-walled bell peppers like you find in Japan and China. Even the local Asian grocery stores don't sell them, and I can find pretty much anything else.

Green curry should have Thai eggplant yet I can only get it with that from one Thai restaurant I know

Fiddleheads

Any of them before soil depletion and banana blight. Fruits and veggies tasted so much better in the 80s. Melons in particular taste lifeless now. Once in a while I strike gold at the local farmer's market or in our own garden.

Pineapple guavas. I can get them where I am sporadically, but they don't ship incredibly well.

https://en.wikipedia.org/wiki/Feijoa_sellowiana

Feijoa sellowiana[2][3] also known as Acca sellowiana (O.Berg) Burret,[4] is a species of flowering plant in the myrtle family, Myrtaceae. It is native to the highlands of southern Brazil, eastern Paraguay, Uruguay, and northern Argentina.[5] Feijoa are also common in gardens of New Zealand.[6] It is widely cultivated as an ornamental tree and for its fruit. Common names include feijoa (/feɪˈʒoʊ.ə/,[7] /-ˈhoʊ.ə/,[8] or /ˈfiːdʒoʊ.ə/[9]), pineapple guava and guavasteen, although it is not a true guava.[10] It is an evergreen shrub or small tree, 1–7 metres (3.3–23.0 ft) in height.[11]

Ripe fruit is prone to bruising; difficulty maintaining the fruit in good condition for any length of time, along with the short period of optimum ripeness and full flavor, probably explains why feijoas are not exported frequently, and are typically sold close to where they are grown. However, intercontinental shipping of feijoa by sea or air has been successful.[10]

Because of the relatively short shelf life, storekeepers need to be careful to replace older fruit regularly to ensure high quality. In some countries, they also may be purchased at roadside stalls, often at a lower price.

Feijoas may be cool-stored for approximately a month and still have a few days of shelf life at optimum eating maturity.[10] They also may be frozen for up to one year without a loss in quality.

Nashi, or Japanese pears. Had some in Japan last year, and they were fantastic. Texture more like a soft apple, taste was great.

I'd love it if Stephen Hawking and Freddy Mercury were alive again

After having açai I wish it was easier to get those than blueberries. They're basically the same, but they actually have flavor without needing to be turned into a sauce or jam.