Looking good.
For what it's worth...
Filters are directly related to fish load and more specifically the amount of food you feed them. For 99% of water gardens like yours no filter is required. I should say additional filter. Your waterfall/stream is a filter. And it's somewhere in the neighborhood of 10 to 30 times more efficient than any submerged media filter for converting ammonia (fish waste). The wall of the pond also acts as a filter, meaning a place for ammonia converting bacteria to grow.
So unless you're planning of adding hundreds of large goldfish on day one I wouldn't worry about filters. People will say "yeah, but goldfish grow and reproduce so you'll need filters". Ask them how big a filter? They have no way of knowing. If you feed 100 goldfish 1 oz of food a week you'll measure no ammonia. No ammonia means no additional filter is needed. If you instead feed 2 lbs of food a week you may measure ammonia and additional filters would be needed.
I most commonly read people want to be "safe". There is one and only one way to be "safe" and that is to measure ammonia occasionally, or more often when a big change is made. To think adding a filter is somehow "safe" is misleading.
After ammonia the next biggest concern is green water. Simple 100% effective solution, UV filter. Properly sized to your pond and pump. Not a combo filter/UV, the filter part is worthless. Maybe a nice 25 watt deal for a around $200. However, there's every chance your falls/stream will be able to support bacteria that kill green water algae. Plus, if you add plants it's likely string algae will come with them and also attack green water algae. But this is a battle, the green water algae have defenses. If after maybe a month or two of green water I'd say the green water algae won and bring in the UV. The UV will kill the green water in a few days....which provides food for the green water killing bacteria and removes the chemicals produced by the green water algae that was attacking the bacteria. This can allow the bacteria to get the upper hand and from then on it will keep the water clear. So maybe after a couple of weeks you can turn the UV off and see if the water stays clear. Some ponds do need the UV to always run, but most don't. If your pond stays clear you can even disconnect and sell the UV getting most of your money back.
Bottom drain, you're right, difficult to connect a skimmer and bottom drain. Also difficult to run both on one pump.
One thing I see missing...there has to be some way to push stuff to the drain. A drain will only pull an inch or so around the hole. Couple of ways. Some drains come with a place for air to come out of the center of the drain. Another is placing Tangential Pond Returns (TPR) which is just a fancy term for directing pump output at an angle so the whole pond swirls. This can also be done with a catch basin for your waterfall. Water exits at the bottom in a direction the swirls the pond. The catch basin also keeps your surface still for better fish viewing and also collects foam instead of seeing that floating around.
The final third of a bottom drain system is removing the stuff. Several types of filters do this. Another option is no filter. In this case you open a valve a water from the bottom drain rushes out. You close the valve and replace the lost water. Your pond acts as a settling tank. This is good when you just want to remove some muck build up.
In this size pond I'd probably skip the bottom drain altogether. Use a leaf rake (net from the swimming pool store) a couple of times a year and you're good to go.
All depends on what kind of pond you want. If you want a really heavy fish load then a bottom drain connected to a filter is a good idea.
Skimmers are always good idea I think.
Cheap and easy so why not.
Additional filter space could be had in the front as you said. Probably the easiest. You can also dig out that corner where the waterfall all the way down to the deepest part. Then lay the liner, build a concrete block wall on top of the liner where the waterfall wall is now. Fill behind the wall with gravel up to water level and build your waterfall on top of that.
I'm not a fan of plant shelves...but it's not my pond. Instead you can enclose the shelves.
Lots of ways to do this, but you get the idea. Those beds can be filled with gravel and become a bog filter. You pump water in and it flows thru the gravel all the way around the pond to a low spot in the wall. That would give you maybe a 50' or longer gravel bed. And you can plant in the gravel so you still have what the shelves were for, but way better. Since there's no standing water in the beds you get zero string algae in and around the plants which is a huge mess. Raccoons won't knock pots off shelves into the pond, huge mess. Wind won't blow over plants, so you can grow much taller plants like Canna.
