I like the salvia that blooms with long purple spikes, not too tall. Both salvia and sweet taters like the heat, so I think they would do well together.

The one I use is called 'May Night'.
If you want a contrasting color, maybe some rudbeckia

Anything with brightly colored blooms in pinks, reds, orange. yellow or white would pop against the dark purple leaves. Such as Lantana with orange or yellow blooms, or 'Sun'patiens with pink, magenta, or orange blooms would look good with the SPV trailing undereath them. Even the Knock-Out roses would look awesome.