I think it's unreasonable to think you'd need to go ALL the way down to original grade with fence posts in order to have a sturdy fence. What you do hinges on the strength of the wall which, of course, we can't gauge from afar. But it appears to be a solid, poured concrete wall, so likely it's strong and adequately stabilizes the soil for a fence erected nearby. In spite of that, it might be as much questions about other things that guide what you should do with the fence. The first of those questions would be appearance. Consider extremes in order to help determine what will be good. One extreme would be a fence directly on top of the wall, extending its plane. Since the retaining wall is a plain, unadorned, flat surface and a "modern" fence with horizontal elements is likely to look similarly hard, raising the height in the same plane is likely to start looking like a military installation, sending messages like: keep out ... off limits ... secret ... Bin Laden used to live here ... etc. To the contrary, stepping the wall back a few feet, is going to give a friendlier appearance. The next question to consider is the construction of the wall. A good retaining wall is built with a layer of drainage gravel directly behind it. Water is collected at the bottom of that layer and piped out from behind the wall so as to prevent pressure pressure from building up (and trying to push the wall over.) In the pictures, a border of gravel is seen at the surface of the back side of the wall. I'm thinking this is probably an extension of the drainage gravel all the way to the surface. Whether it is or isn't, it would be a good idea not to place the fence posts directly behind the wall where they would be in the gravel layer. Thus, there are two good reasons to step the fence back from the wall face. Given the scale of the property, I'd say a 3' set back would work well. I wouldn't go less than 2'. Might go 4'. If you want more specific advice, show the property from the curb, but lined up with the center of steps, and take slightly overlapping pictures that shows all the way from the left neighbor to the right neighbor ... so we can see the complete scene. Also, show a plan view of how the retaining wall is laid out on the property. I am not against the "modern" look at all but think you must consider how it integrates with its surroundings. It might be more modern looking if all the plants on and surrounding the property were vaporized and one created a look along the lines of a power company sub station ... like a desert of gravel and an engineered looking patch of grass. Hmmmm .... maybe that would be a little unfriendly looking ...? Actually, I think your modern-looking house looks fine with the trees and greenery around it. SHADE is a nice thing ... more friendly than blazing sun. Along these lines, I'm thinking that maybe extending your retaining wall with another hard, fairly plain surface, is going to be just too much hard and plain. Personally, it would be for me. Because something has a friendly surface does not mean it contradicts compatibility with "modern." To the contrary, I think a friendly surface has the ability to HELP modern look better. One such example that comes to mind is how creeping fig can cover a wall in greenery, while at the same time looking very "modern." Here are examples (one of them doesn't qualify as modern.) I'm not suggesting creeping fig, but using it as an example of a "look" that is compatible with "modern" but still not unfriendly. Even if a "hard" fence is used, I think there must be something interesting and friendly about it.

Take a photo of your fallen wall and post it here. When the builder is building the new wall with stacked concrete bags take more photos. I've never heard or seen his solution before. His corrective measures are a long way from good engineering as I know it. Also, I've never, ever heard of a wall falling down from the very first hard rain. That staggers me. While there are differences based on product, height and existing soil types here is your basic retaining wall formula: A. Compacted CA-6 gravel base. B. Level wall construction within the stated limits of the specific wall product. C. Roughly 1' of free draining CA-7 gravel behind the wall. D. Perforated drain tile behind the wall (probably) with the tile "day-lighted" through the wall face if the wall is tall enough. E. Geotextile fabric as a separator between the free draining gravel and the existing subsoil. It sounds like he is going to use the concrete bags sort of like a C.M.U. substitute or as a substitute for the free draining gravel. The CA-7 gravel is what reduces the surcharge, which is what you want to avoid with wall construction. If soil is behind the stacked bags of concrete then the surcharge will be against the stacked bags, eventually pushing the wall forward again. What area of the country are you in? Do you know the name or manufacturer of your wall product? If you do, call the manufacturer immediately. The manufacturer has a vested interest in their products being installed correctly and will normally have field reps available for just this type of thing. Again, post some photos in this thread tomorrow.

If you have any paving, Carrotwood and Podocarpus are both pavement lifters/busters. Podocarpus is one of my favorite trees, but they have massive root systems. Italian Cypress, but planted strategically to screen your neighbor's windows, rather than as a "Fort Cupressus" hedge. No bees on those. The Sunset Western Garden Book has a "What to Plant Near Swimming Pools" section. Every library in California usually has a copy if you don't want to buy one. The fruitless olive still blooms, it just does not produce fruit. The flowers may still attract bees.

We live in WI, same zone and are about 5 .years in of converting several large over grown beds into native plantings. We dug a lot out and then went with the smothering route for a lot of it because we had somewhat contained yard areas of ditch lily that were just so badly overgrown. My personal opinion is to agree with Christopher C NC. I think he might of said it best, but if it's helpful I can add what I can. Smothering and chemicals do not seem to be practical with what you want to do here. It's a big commitment to kill off everything and start over, without ending up with just as many or more of what you wanted to get rid of to start with. There are plenty of natives that will love to grow in that soil as is. Things like common/wood violets, goldenrods, big leaf aster, golden grounsel, black eye susans, milkweeds, butterfly weed, etc should have a chance to hold their own, and the bees and birds will benefit a great deal. There are also native shrubs that could work and be beneficial. Someone mentioned Joe Pye weeds and culver's root which might work nice for you also (our rabbits love the joe pye weed when its first sprouting so we don't have as many. They also ate all our cone flower) Just go in and carefully pull out the things like garlic mustard and buckthorn, that offer zero value to wild life and that smoother beneficial plants. You can poke some holes and plant specific plant seeds. Plant just a couple kinds so you know what you're looking for as it grows, or just plant a bunch and forget it other then the worst of the bad weeds. Or if you start the seeds indoors and grow them as plugs you can use this drill attachment to make plug holes to pop them in. (I watched a master gardener use this to beef up her plantings around a pond, She said it helped by not disturbing the rest of the soil, which can encourage weed seeds to grow.) https://powerplanter.com/? keyword=power%20planter&matchtype=e&sncid=14&utm_source=google&utm_medium=cpc&utm_campaign=brand&adgroup=brand&gclid=CjwKCAjwvtX0BRAFEiwAGWJyZPjlsjDZNxN_E-CX_Ium7Ki2s5PoqBvs2ZW6QwHL0YKaNRU1RGPYZBoCpK8QAvD_BwE I've bought seeds and flats of plugs already grown from Agrecol native nursery in Wisconsin. http://www.agrecol.com/ and Prairie nursery https://www.prairienursery.com/store/groundcover-plants/page/1?_url=groundcover-plants/&_page=2 You can also check out the local chapter of https://wildones.org/ for tips and resources. They have a seed exchange met up, I think in the fall. (new members encouraged) This might also be a useful book if you are still learning invasive species called Invasive Plants of the Upper Midwest: https://www.abebooks.com/Invasive-Plants-Upper-Midwest-Illustrated-Guide/30434932321/bd?cm_mmc=ggl--US_Shopp_Trade--used-_-naa&gclid=CjwKCAjwvtX0BRAFEiwAGWJyZLZ0HNTSSa2ynRehaB0ezU1hRmI0oQoPGtXfDzjMegRobsoPz2AtkBoCHUMQAvD_BwE A link for some other suggestions of hardy natives written by Johnson's nursery expert: https://www.kb.jniplants.com/naughty-natives-part-3/ If you go on the midwest garden lovers group, or anywhere else, be careful of what you accept as free. Be careful not to introduce things like snow on the mountain, orange ditch lilies, etc or invasive bugs/worms. Lastly if by chance you are in our area in SE WI and feel like you need a weed wacker, we might have an extra you can borrow. I also agree, you are giving back the land a great gift.