Nāgārjuna is definitely a Mahayana figure, but most of what he had to say is still very relevant to Theravada practice.
As for Buddha-nature, you might be interested in In short though, there is no concept of Buddha Nature in Theravada.
Gain and loss, status and disgrace,
censure and praise, pleasure and pain:
these conditions among human beings are inconstant,
impermanent, subject to change.
Knowing this, the wise person, mindful,
ponders these changing conditions.
Desirable things don’t charm the mind,
undesirable ones bring no resistance.
His welcoming and rebelling are scattered,
gone to their end,
do not exist.
- Lokavipatti Sutta