The COVID-19 pandemic saw the first extensive use of adenoviral vector vaccines, with over 3 billion doses produced during the first year of the pandemic alone and an estimated 6 million lives saved. These vaccines were safe and effective, and could be produced at low cost in several continents allowing widespread use in low- and middle-income countries (LMICs). Despite their successful deployment against SARS-CoV-2, their impact has been overshadowed by relatively lower immunogenicity in contrast to mRNA vaccine technologies and very rare but serious adverse events such as vaccine-induced thrombotic thrombocytopaenia (VITT). The next-generation of adenoviral vector vaccines must address these challenges: here, we explore strategies to improve immunogenicity and safety by novel serotype selection, vector engineering, capsid modification and new delivery technologies, and discuss opportunities for next-generation adenoviral vectors against infectious disease and cancer.