Are there any DC characters that truly feel "outdated"

[Aahz]

They have in general problems to bring any new characters up in popularity. Unless they connect the character to one of the big six franchises.
And that has like you said the problem that they will not be the main guy on the long run, that there franchises have now already that many characters that it is really hard to add new ones.

The few new characters that succeed in the past decades, were usually the ones that were pretty different from the existing ones.