Lets try thinking out of the box for once.
So, here is a classic Victorian garden design, formal layout, geometric patterns, usually a tropical focal point collected from somewhere in the Empire at the centre of the display. I think a lot of us saw this in City parks or come from a Victorian mindset whether you're aware of it or not! There is nothing wrong with Victorian gardens, they are amazing, but unless you can carry off your dracena and geraniums to this level of display lets go with something more modern and pleasing.
I always like a few annuals, in an area where we have a three month growing season I like to enjoy the season as much as possible. One of my favourite annuals is Ricinus, the big leafed plant in the middle, it gives that need for height and a nod to Victorian tropical interest. This particular pot also included coleus and nicotiana. I don't love what I call "floral vomit", where every colour possible is included in one pot, you know like the kind that line the streets of Victoria B.C., sorry Victorians!
I go for just a couple of contrasting leaf colours and maybe one colour of flower.
Here's one from a few years ago, the dark leaves of ipomoea or potato vine contrast vividly with the chartreuse of this coleus. A few silver leafed annuals balance the equation. Of course I took a few courses in colour theory while in Art School so types of contrasting colour combinations come quite naturally to me. Pick whatever you like just try to limit it to one or two colours. Maybe you like yellow? Purple is it's opposite, highest contrast. Yellow and red would be hot colours, very vibrant, for example. Try putting some annuals together while you're at the greenhouse and see what you like. And lets try something new! There's more than lobelia and geraniums out there these days!!!
This was one of my favourites from last year! When buying annuals for pots also consider location as in is this plant suited to a hot and sunny location, shaded, half sun? You may want to ask at your local greenhouse or pay close attention to those plant tags. This pot is for a partly sunny location, morning and late afternoon sun.
Remember that annuals in pots need to be watered at least once a day and fertilized every other week.
A benefit of annuals in pots is they can be moved if there is hail or frost and you can rotate out one batch for another as the season goes. I usually start with pansies in April, move to a summer annual until September and finish with kale in the fall.