It could work, but it's not perfect.
Wood ash has no nitrogen and sulphur and is rather low in phosphorus because much of it is lost during combustion. It is also extremely alkaline so you have to do some maths to avoid over-applying it, though dilution with lots of water and using the mixture to irrigate your garden after adding lots of organic matter should work.
Legumes can add nitrogen to soil, but it's not guaranteed unless you inoculate the seeds with the right bacteria before planting. And even in that case, I feel it takes too long because a legume plant's fixed nitrogen isn't added to the soil until it is killed and starts to decompose. Urea is more cost-effective as a nitrogen source.
Leaves would add organic matter, but organic matter in a fertile and moist soil gets eaten up pretty quickly by worms, microbes etc, so you would need to keep adding the leaves regularly to keep it up. Living and dead roots in the soil are the best and most sustainable source of OM.
My preferred soil building approach is to cover it with plants (highly productive cover crops, or what you actually want to grow) and fertilize them with a balanced commercial liquid plant food or soybean meal, but the fastest shortcut to good soil is to buy lots of compost and mix it into your soil. It provides organic matter and, if added in large quantities, nutrients, too.
Questions to improve my answer: What kind of soil do you have? Why do you feel it needs improvement? What do you want to grow on the soil when it's ready?