After the Mangekyou and Eternal Mangekyou phases, one might wonder if it's possible to upgrade the Mangekyou to upgrade beyond that. I think that once all a Sharngan's levels have been maxed out (aside from Rinnegan, which only Indra's reincarnation can awaken), there is at least a possibility that the Sharingan can be further upgraded, at least according to Sakura Hiden. Granted, I have made my disdain for this novel known in the past and I still don't fully trust it as a canon source, so readers can take this segment with a grain of salt.
But in the light novel, Kido was convinced that killing Sasuke's friends and making him suffer would further increase the power of his Sharingan eyes. Now how Kido would know this, I have no clue, so Kido may be hypothesizing rather than basing his theory off of any actual evidence. And there's a strong probability that Kido's speculation completely misses the mark, and that in reality, once a Sharingan's levels are maxed out with Eternal Mangekyou and Rinnegan, it can't be improved or upgraded further.
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But seeing as how the Sharingan levels up and thus becomes stronger in response to emotional pain, grief, fear, sadness, guilt, etc., it seems reasonable to assume that even more suffering would continue to further refine and improve an Uchiha's vision and visual prowess even if it didn't level up, because the Sharingan works by being upgraded in response to its user's pain, so perhaps even if it doesn't fully level up to a higher phase, more suffering still boosts its power.
As for the question of if there is another Sharingan phase beyond the Eternal Mangekyou Sharingan, I have to say no, because no Sharingan phase was shown beyond the Eternal Mangekyou Sharingan in the series, meaning that if there was another Sharingan phase after EMS, it would require situational circumstances that are even more specific than those required to awaken the MS. That means that the situational requirements would be so specific and impossibly unlikely that virtually no one would be able to fulfill the requirements anyway. So we're just going to assume that any theoretical additional Sharingan phase doesn't exist, since the story never said otherwise, so we can safely assume we have seen all the Sharingan levels from the Uchiha clan.
