"Domesticated" is an odd term though, I wouldn't count something that is simply edible or "foraged" to be domesticated for example, they need to be sellable, easily prepared. We probably need something that can actually be marketed to the general if we are making new things or trying to introduce healthier foods to the public.
Lardizabalaceae seems like a decent one. I wonder if some of the species are close enough to attempt crosses? The pulp seems to be the only edible things in this family though.
Lonicera villosa is native to North America and apparently edible. Seems similar to Lonicera caerulea. The only other "edible" honeysuckle that I can recall at the top of my head.
Amelanchier species could be worked on a bit as well. Seems to be in the same boat with Medlar though, probably needs to ripen for awhile, as a result doesn't last long on shelves. Also the almond taste you get from them is cyanide - or rather your body makes it into cyanide. That could be touched up on a bit.
There are a ton of Vaccinium species similar to blueberries that could be worked on. Lot of diversity there, the problem is that most are sort of bitter / tart. Suppose people could attempt hybrids if they wanted to... No idea how well it would work.
Ribes could be cultivated a lot more than they currently are. Higher shelf life would be a big thing to look for. Jostaberry could be something nice to start off with for hybridization. Unsure if it could be a "easy" bridge between European and north American species.
Good bit of Prunus species could be domesticated but need improved shelf life. Maybe more hybrids could be attempted.
Lycianthes can probably be hybridized and used to make better domesticated crops.
There are a bunch of Cyphomandra species that are somewhat cold hardy - one is hardy down to 15F. These factors would greatly increase the market, there are also species with larger fruits. Longer shelf life would be nice as well.
Jaltomata have a wide range of flower types, colors, fruit color, taste, fruit size. So there is a lot of potential there. They are already selected out from common weeds by farmers.
Asimina or Annonoideae in general could be improved on. There are dwarf Asimina species, shelf life and flowering time need changed up to really make it marketable as its only available for a few weeks or so. Desert, dryland species exist. The regularly grown species loves water and moisture, shade. Also it is usually used when ripe or overripe so that might be a negative.
Carica is a pretty broad genus, some species are decently cold hardy for "papayas".
And yeah Urtica is a huge genus. Lot of nettles that are edible, some don't sting as much or at all. Some have natural variegation. Leaf size and shape seem to greatly vary as well.
Rheum has a ton of "edible" species. There is already a hybrid on the market, so making more should be possible. Finding a species with really low oxalic acid would be nice. Maybe it would taste edible, leaves and all. A lot of them are already decently cold hardy as well.
Quercus is an interesting one. There are already a bunch of hybrids available within groups. Quite a few have nicer wood, acorn size, sweetness. Could probably be improved on - stabilized a bit so that grafts aren't overly used. Wouldn't want a newly domesticated oak to be a large disease site in the future...
Castanea and Juglans could be further looked into as well.
Chenopodium species are already edible but might need further selections. Native Americans used these a good bit. The types they grew might be lost though. So we might have to try and recreate some.
Melothria is a decent genus as well. Could probably be added to salad mixes found in stores. Again maybe shelf life? They seem to last a good while already for me though. They can get a bit sour/bitter if you let them go without being watered. So that might need touched on. Oh yeah and one species is a laxative when overripe.
