I have some in a 3 gallon container, they never look bare as new shoots are always coming up. If yours are sparser, perhaps something trailing like a creeping jenny would look nice and add visual interest.

I was wondering about some lobelia.

the canna will crowd out whatever you plant with it...possibly put a low grower in a pot in front of the cannas ??

I planted Bakoppa (sp)beneath one of mine. It has filled the surface of the soil around the Canna and is hanging over the edge. It looks great. In another I have Blackie, a Sweet Potatoe Vine. It has done very well also spilling out over the edge of the pot, and the colour looks great with the green Canna leaves above.
Just choose something that has the same water needs and will spill out of the pot instead of growing tall.

Companion planting is more than just plants that look together. It is also about plants that support and look after each other.

Aphids detest Lavandula, and so lavender's make a great companion planting for any plants subject to aphid prevedation, like Canna.

Canna has no scent, and so plants that are heavily scented, such as roses, thymes and lavender, make great companion plants on a scent basis.

I think that you can get the idea from those examples.

Regards,
Malcolm

I am late summer is almost over. I grow my cannas for the leaves. So most of my pots have the Cannas with interesting leaves.

Tropicanna, has (Dahlia Bishop)with dark leaves and red flower, and coleus all around the bottom.

Bengal Tiger, has coleus in front and because BT is a short Canna I put Glads in the back that bloom yellow and Red.