I always wanted to test whether wild plants alone could be enough for a wholesome diet. This session was a perfect opportunity to (at least theoretically) do it. The short answer is - yes, they can.
I gathered the data about nutritional composition of various common wild edible plants, mushrooms, and algae and composed this data table (you might need to download it to be able to access full information in each separate cell). For each essential nutrient listed in this session I named two or more wild edibles that are rich in that nutrient enough so that one would need to consume low to moderate amounts of the plant to reach the daily norm of that nutrient. Exact amounts of the nutrients per 100 g of each plant/mushroom are given in the table along with the links to the data sources providing that information. I also added total energy value (kcal), carbohydrates, and fat to the initial list according to this online calculator (assuming the "average" 30 y.o. male with a height of 180 cm and weight of 70 kg).
All the plants, mushrooms, and algae in the table have a wide distribution area either in Eurasia or North America, or both. However, each specific geographic area where certain individual lives would require separate desk research to cover the nutritional spectrum with local wild edibles. This attempt was just to prove that it's relatively easy to find common wild plants that would fill all the nutritional needs.
Such a table could further be expanded including more and more plant, mushroom, and algae species making it an evergrowing database and then connected to wild plant utilization app. Such technology would let any person residing in any natural environment know which plants to collect to provide themselves with a wholesome wild (and free) diet.
Naturally - the warmer and more humid the climate, the easiest is to depend on wild plants. Places with some evergreen flora available all year round are the best for this. It is nevertheless possible in regions with expressed winter season, but it would require much more planning and relying on the preservation of foraged goods.
Proteins and essential amino acids
In the table I placed special emphasis on proteins because they are often considered difficult to obtain from a plant-based diet let alone a plant-based wild diet. Therefore I provided more wild edible plant (and a couple of mushroom) species for protein category than for any other nutrient category. Concerning essential amino acids, the same rule that is valid for general plant-based diet is valid for wild plant-based diet - you have to have two or more different plant protein sources, so if you are eating nuts, you should also eat nutritious stems or underground parts of other plant or/and grains of another.
Judging from a few articles that did provide data about the exact composition of amino acids of certain wild plants, those plants that have high protein content usually have good amino acid content as well and if few are below the norm in a certain species they can be obtained from other wild plant sources. Mushrooms are a good option for non-animal proteins, differently than plants, the same species tend to contain full spectrum of all the essential amino acids. That's one of the reasons I included them in the table.
Vitamin B12 is the toughest nut to crack when it comes to plant-based diet. The only truly reliable natural food source of it seems to be sea algae. Different articles tend to emphasize different algae species, but one genus that is generally agreed to have sufficient amounts of B12 is Porphyra. Porphyra spp. are distributed worldwide. Sushi sheets are made from algae of this genus. Consuming few grams of dried Porphyra algae should be enough to meet the daily norm of vitamin B12. However, if you live far from the sea, wild foraging of sea algae is not an option.
Alternatively, some mushrooms, namely shiitake mushrooms, golden chanterelles, and black chanterelles have been reported to contain considerable amounts of B12. But to obtain sufficient amounts of this vitamin from them alone a person would have to consume 50 -200 g of dried mushrooms per day, which is practically infeasible. However, they could serve as a partial supplement for someone who loves mushrooms.
Seaweeds are also an excellent source of iodine, some containing few or even tens of times the daily norm in one gram of their dried mass! For those who are not fortunate to live close enough to the sea, it's best to get both iodine and B12 from supplements. Iodine is easily obtained by eating iodized salt and for B12 you'd have to buy commercial supplements, but even then this sort of wild plant food-based diet would still be super cheap moneywise.
Wild edibles are not the easiest way to feed yourself, but it's definitely the cheapest (free) and the most off-grid one. It takes just as much (or arguably even less) knowledge and skills as hunting and fishing and doesn't require killing wild animals.
In addition, wild edible plants often contain bioactive compounds that are beneficial for health - flavonoids, antioxidants, anti-inflammatory compounds, etc. Plants growing in the wild tend to have higher concentrations of these compounds, because they are exposed to harsher environmental conditions, than garden plants. Plant food that is bought in mass supermarkets has the least, if any, of such compounds, because the growth conditions for crop plants are made as favorable as possible in order to maximize nutritional value and attractive product appearance.
Generally all well-known (enough information about plant edibility from various sources should be available) wild edible plants and mushrooms are safe to eat, but one should study the details about consumption of each particular plant/mushroom. It's often safer to eat thermally processed wild plants since toxic compounds are usually broken by heat, but many wild plants, especially leaf vegetables such as ground elders, lamb's quarters, and sorrels can be eaten raw without any danger. Sometimes only specific parts of the plant might be edible, while others toxic (e.g. bird cherry) or a plant might require specific processing to be edible (e.g. stinging nettles). However, with some dedication and practice the necessary knowledge and skills can be gained.
When picking wild plants it's also important not to devastate wild resources, a good rule of thumb is not to use more than 1/10 of a population of specific plant species. If there are 10 plants you can pick one, if ~ 100, you can pick ~10, etc.