Urban mobility systems are increasingly exposed to complex, interacting disruptions that challenge conventional resilience-based planning approaches centred on recovery to pre-disturbance conditions. Antifragility offers a complementary paradigm, emphasising learning, adaptation, and performance improvement under volatility, but remains weakly developed in mobility decision-support environments. This paper positions an ontology-driven requirements framework and a Delphi-inspired expert consultation as core instruments for specifying antifragility-oriented capabilities for the Simulator for Urban Mobility Antifragility (SUMA), developed within the Horizon Europe AntifragiCity project. Building on a systematic literature synthesis, project deliverables, pilot-city inputs, and a structured expert scoring exercise, the study focuses on the consultation process used to validate, clarify, and prioritise a solution-agnostic requirement set spanning functional, data, interoperability, governance, and equity dimensions. The paper demonstrates how ontology-based structuring and Delphi principles can be integrated to elicit, validate, and prioritise stakeholder and expert judgement, contributing a structured and replicable methodology for translating antifragility principles into implementable, consensus-informed requirements for complex socio-technical digital platforms.