Some suggestions would be yarrow, milkweed/butterfly weed, coneflower, liatris, bee balm.
Park Seeds (and I'm sure many others) carries seeds for all of these. They mostly come in colors to complement or contrast with your rudbeckia and are in a similar size range. The ones I personally have started from seeds in the past are liatris (easy!), yarrow, milkweed.
But I start seeds indoors under lights. Perennials can be slow starting from seed, with longer germination times and slower growing, especially in the beginning. They can be started in the ground, but these cold hardy perennials do best planted in fall to over winter, because they need the cold period. You could try planting these seeds outdoors in early spring as soon as your ground can be worked. Or you can try winter sowing: https://www.bachmans.com/files/GardenGui ... gGuide.pdf
Some people even just toss seeds on top of snow and let them work themselves in as the snow melts. I have never tried that and can't speak to it.
(Especially if you are not real experienced with starting things from seed) easier would be to plant some annuals, with / around your rudbeckia. Marigolds would be one very easy one to start from seed in the spring. You can get marigolds the height of your rudbeckia or you could get dwarf ones to go in front. The marigolds will bloom and bloom all season and are quick to bloom from seed.