Why Ekko will Always build tank, Only solution:
Builds tank because his kit is designed that way, independently of his scaling.
He is not an assassin because he lacks the burst and hard-to-dodge/point-and-click spells main sources of damage that actual assassins have (his Q-W-R are probably the easiest to dodge spells in the game)
Counterbalancing that, his passive and W passive are his reliable sources of damage, which require extended battles at melee range.
So ye, it's not a matter of stats, it's a matter of his kit, which will make him always worse as pure damage.
Only Solution: The only way i see him actually becoming an assassin, is if they increase by a whole bunch his Q speed, decreased W delay and make R targettable. With him having reliable sources of damage, they could tweak his scaling and balancing him as an assassin if thats what they want.
and
are both ranged, while