I've been clicking through to those Numantia articles, thanks for gathering the pics! Looks like a smashing setup.
I too would like to see a Celtic theme but looking at the latest Romans I am not confident they would do a good job.
Anything would be better than those "Gauls" they did awhile back. The Romans were excellent when first released. It's only not so good now because they've decided to create new sets by recycling old parts without knowing what the original designs were meant to be.
It wouldn't require a lot of new moulds either.
One sword with lobbed pommel.
One spear with wavy blade.
A shield with central spine.
Helmet - coolus type though an agen-port type would be nice too.
Neckring - clip on.
Heavy swept back hair style.
Carnyx - war horn.
Four lobbed saddle which could then be used for Romans as well.