Novgorod had an inherent weakness of not having reliable grain source within it's domain, therefore it's subordination to whoever was controlling central and southern part of the Russian plain is kinda predefined. But, if you want to be adventurous, one way to achieve Novgorod's supremacy would be making Southern trade (Kiev rose on racket of Slavic trade wuth Constantinople and Byzantine Empire in general) less profitable than Northern (Novgorod being main extortion point on that road). Novgorod have an advantage of potential control over Eastern route (through Volga) too. Therefore you'll need to create real mess out of Southern Europe around 800 AD (to make Southern trade unattractive and starve Kiev) and (ideally) somehow solve inherent problem of the Volga route being too susceptible to nomad attacks. Probably some form of alliance between Novgorod rulers and Khazars (who more or less controlled Volga at this point) to finish Kiev off would help...