Many theories beyond the Standard Model (SM) predict new heavy particles that would preferably couple to top quarks. In several direct searches it is assumed that these particles decay to final states with a pair of top quarks. This results either in various signatures with top-quark pairs or in a resonant distribution of the invariant mass of the top-quark pair. In indirect searches it is assumed that SM processes might be affected by massive new particles, such as the enhancement of the rare SM process of four-top-quark production. The results from latest searches, performed by the ATLAS Collaboration with 13 TeV data, for vector-like-quark pair-production, for resonances that decay to a top-quark pair and for four-top-quark production are presented.