Here's my advice for people getting into stage design:
Start with 1-3 concepts that you want to include in your stage, and then fill in the rest around those concepts. Don't sacrifice or degrade those concepts in order to hit round count or otherwise do other things you think might be good - build around the concepts.
What do I mean by concepts? Think of the most interesting parts of stages you have shot. Some choice that was presented, some technical sequence of timing or movement required, some risk-reward offering.... anything that was worth discussing with other shooters during a walkthrough or, better yet, after the match. If your stage has even one really interesting component, it will be better than 80% of the stages in most matches. If you have 2 or 3, you probably have a really good stage (assuming it is rule compliant and safe otherwise).
Example: After 6 months off from doing stage design, I volunteered to do a couple of stages last night for the indoor match I ran for several years. I had 2 concepts I wanted to include in our larger bay, both of which are slated to appear on multiple stages of the upcoming state match: 1) a mini-popper activated swinger not available from the same location as the mini-popper, but only 1-2 steps away (thus rewarding a shooter who can call a shot on the activating popper and move immediately a short distance to catch the first exposure of the swinger); and 2) a "flowable" stage section where targets of 7-10 yard range present and get lost within a step or two of movement, but not so densely packed as to demand static shooting (to reward shooting on the move).
So I first built the starting position around an array of steel including the activator, and separated it with walls from the swinger's location. I threw in an extra paper target in the vicinity of the swinger to give people something to do if they missed the timing on the swinger, but didn't want to add any more because that would so fully "cushion" the swinger's availability that nailing the sequence wouldn't reward the shooter who pulled it off. So there's concept #1.
Then I built a walled off area with targets available through a port at the other side of the bay (and significantly downrange). The port array wasn't one of the concepts, but it was used to give a spot shooters had to reach by the end of the stage (in front of the port). Along the way, I used a couple of walls and a barrel stack to have 3 targets that were available during a 10-foot stretch of movement, but making sure that there was no shooting location where all 3 were available. There was concept #2.
At that point, I had a couple of steel and 5 paper targets out. I added another target that was only available from a mid-point in transit from the first concept to the second, and allowed one (but not all 3) of the concept #2 targets to be engaged from that same area - although from a further distance than if the shooter tackled them during the concept #2 movement. Since the stuff behind the port was somewhat isolated from the rest of the stage, I threw in a mix of partial and open targets at different heights for some not-completely-vanilla blasting at fairly close distance. Done.
Was it a great stage? Not really, but it was interesting enough that those who went out for dinner afterward talked about it at some length. There were big differences in time and HF for those who executed well versus those who did not. Nothing about it was super difficult (although a 12-15 yard swinger is not a joke for most of us, including me), but it was an effective "separator" in terms of rewarding skill and execution. Other than the port, there was some room to "float" on most of the targets, so it felt freestyle and didn't force a go-here-shoot-x-go-here-shoot-y approach. There were options on where to engage a couple of targets and options on how to attack the activator sequence. It wasn't larded up with a bunch of meaningless targets that diluted the core concepts so much that fouling them up would be washed out by a bunch of hosing, but there was also nothing punitive about it. It was the kind of stage that I like, anyway.
So there's an explanation of one approach to designing stages. There are certainly others.