Is there room for an ornamental grass?

hmm-

the rhodie flowers first, then the rose...

how about a bank of tall lillies? I have the old fashioned orange turk's caps standing up through a bed of irises and daylilles, and they make a great vertical statement.

either holly hocks or Lupines are another suggestion- again, a great vertical statement, and the lupines are about mny favorite flower in the whole world for the smell alone!

and I'd be tempted to put some annuals- verbenas, or allysums in the very front of the bed.

(actually, if you can find it, there is a perennial alyssum that makes standing bouquets of tiny yellow flowers that is one of the best foils for every other flower in the garden in the spring)

I'd go with a tall purple fountain grass (36"-48" mature). It would add complementary color, form, movement, great companionship behind the coneflower, and a nice change from all the green while you decide a more permanent solution. Is this a butterfly garden?

What is the name of the yellow perennial alyssum you mentioned earlier?

uhm- is just called perennial alyssum. that's all it says on the tag, no latin.