Yes, kagune shape depends on the ghoul's imagination, but it's also determined by strength. A weak ghoul with a powerful imagination will still have a weak and basic kagune, because they don't have the strength to apply their imagination to their kagune. Hence, in TG, a lot of ghouls had kagune that were more-or-less just tentacles or wings sprouting out of their backs, whereas in re: most ghouls we see are capable of making very differentiated kagune. It goes without saying that rinkaku is not an exception to this - the rinkakus we've seen most all originally belonged to Rize, so they do resemble one another at first, only differentiating when the ghouls have reached a certain strength level. There are individual factors at play as well, such as Miza's genetic predisposition toward the three-bladed bikaku and hybrids like Hinami and Shikorae, but I think that covers the basics.