Given that you will at some point be planting in the ground, I would go with things that will transfer. So you get the benefit of them in the pot, but also don't then need to buy again.
I would get a few different hydrangeas. They will flower for ages and then you can transfer them whenever you're ready. Choose pink, green, blue or white flowering. Mop heads or cone heads. The white Runaway Bride was last year's plant of the year at RHS Chelsea, and it flowers all along the stem (hydrangeas otherwise flower at the end of the stem), so amazing in a pot. Little Lime is little and lime coloured, as the name suggests.
An acer would be gorgeous. Seasons long interest. There are some fantastic smaller ones with lovely pink foliage , turning to red/orange in autumn. They are really beautiful.
Summer colour - buddleia buzz will stay small, you can get them in indigo, purple, pink etc. the indigo one is my favourite.
A fuschia, maybe, for summer colour. They're not all fuschia coloured, you can get hardy white ones (Hawkshead) or mauve/white (Delta's Sara).
Patio rose, oleander, liriope all great in pots. Salvias (Hot Lips will flower for ages and grows to a decent size in the first year) too.
If you're looking for summer bedding, you won't go wrong with geraniums, verbena, bacopa, dianthus, petunias etc.