It was said that the elder brother who is the first Uchiha got Rikudou Sennin's eye power which is the Rinnegan and his spiritual energy while the younger got his body and physical energy.Look at the older brothers dojutsu directly:
The spiral can be related more to the Rinnegan design then Sharingan.
My gues is that Rinnegan is just the base do-jutsu similar to Sharingan but even in normal form the Rinnegan is wayyy overpowered.
The spiral partern of the Uchiha ancestor's do-jutsu also matches one of Konohas main symbols,the whirlpool,like here:
The sign behind Naruto
The same sign Konoha jounin wear on their jackets.Is it a coincidence?
I think that the tomoe is the result of deluded Rikudou blood,the Uchihas continue to have a dojutsu form but mainly because of a portion Jubi chakra which gets passed down and not because they have Rikudou's original eye power like the elder brother who probably evolved the Rinnegan.
Many will ask then why did Jubi have tomoes also like the sharingan? We dont know exactly why we saw the tomoe in that image but Rikudou's eye power ,the Rinnengan came direct from the Juubi. The tomoes could represent Rikudou's control over the Juubi, its not that far fetched since a tomoe is also a symbol for the human sipirit.
Genjutsu anyone? So in clonslusion i think the tomoe in the current Sharingan shows the control over the Jubi's chakra that resides in the eyes.Yes the fucking eyes
As Kyuubi said : "Eyes with such power and chakra...."
Now back on the "Mangekyou Rinnegan",if there were such a powerfull dojutsu,what powers do you think it gives the user,considering the massive change from normal sharingan to MS?