I'm no artist, so probably not qualified to comment, but I have had a lot of tanks over a lot of years. I don't really have "display" tanks nowadays, but I still try to make the tanks I keep pleasant for me to look at; and that's the only opinion I care about: mine. Very few people actually see my tanks.
I'd suggest that if you are making a display tankscape, the first thing is to have something as a focal point; a nice piece of driftwood, an impressive rock, a commercially-purchased piece of decor, or an especially beautiful plant. Whatever you use, don't plop it right in the middle of the tank; that always looks goofy. A centrepiece needs to be a little bit off centre, maybe at the one third mark, to not look very artificial and forced. If your tank is much longer than it is tall, then maybe a main focal point and a secondary one would work better.
Symmetry is another thing to avoid. Tanks with plants, rocks, flower pots, etc. placed exactly the same way on both sides never look right.
Plants should be placed so that the tallest are at the back, with successively shorter and shorter species as you move towards the front. But...it helps to place a mid-height plant, or grouping of plants, a little further forward here and there, with shorter ones behind. This tends to increase the apparent depth of field of your vision, making the tank look as wide as possible from front to back. Plants should either be single specimens, like Amazon Swords, or else should be in groups of odd numbers. Don't ask me why, but a group of 3 or 5 plants always looks better and more natural than a pair or a group of 4.
Changing the colour of the background can make a huge difference to the look of your tank. Another way is to simply change the location of your lighting. Just sliding a light forward or backward an inch or two can completely change the entire effect. A single point source of light goes a long way towards creating that lovely shimmering effect, much more so than a diffused light.
Probably the single biggest thing to keep in mind is that you aren't committing to anything particularly permanent. You can expect to be making little tweaks and improvements for a long time, as you home in on the "perfect" look...which, of course, does not really exist, but...
You need input from folks who do a lot of display tanks. There are plenty of them on here, but the one who leaps immediately to mind when discussing beautiful naturalistic designs is
Stanzzzz7
. Hopefully he weighs in here; his ideas won't be much help if you're dead set on a Spongebob Squarepants theme, but otherwise his tanks are truly spectacular.
