You think they can some Native American cultural content? I get there are many different tribes with their own cultures so it would be impossible to add everything; but there is some feedback and suggestions.
Fry Bread recipe
Hopi kachina dolls
Jingle dress (often worn at Pow wows and for dancing)
Adobe and Southwestern style houses and furniture
Maybe certain hairstyles (the Princess Leia wig can sort of work if turned into an actual hair style as it was most likely inspired by the Hopi).
clothing and jewelry with bead work (if that is possible) or turquoises jewelry
bolo ties which can be used by so many people in general, works well for sims living in Oasis Springs
Pottery and woven rugs, baskets, etc....
Even dream catchers that can be used in nurseries like mobiles are and maybe help keep monsters under the bed away (as they are meant to protect from nightmares, so it can add an extra bonus interaction similar to parents spraying the bed.
I am sure others can help give some ideas or tell them what best to avoid (think stay away from warbonnets as they aren't something worn by everyone and have to be earned and things like that, so stick with more common traditional or neo-traditional items as that would be the best way to add things without causing problems); but it would be nice to at least see them try.