I never read about the parallels between Clarke and Lexa as individual characters (aside from the sky/earth business): they are, essentially twins, analogues of one another.  

• – Both are trapped by the circumstances of their birth.   For Clarke, being raised on the Ark means a life of strict adherence to rules – death being the only real punishment.    For Lexa, raised from childhood to be a warrior/leader, her life is also a strict discipline of following rules – death, again, being the only real punishment.  
• – Both Clarke and Lexa, following the death of a leader-figure (Clarke’s father, the previous Commander – both dead by betrayals real and implied) are called upon to lead their people.  In contests of wills and strength, they better their others and show maturity and responsibility that belies their years.  
• – Almost from the moment they assume their roles of leadership, they are again trapped by the needs and rules of their people: if they do not perform as required, they are threatened with crushing judgement or death.  If their choices do not necessarily conform to the wishes of their people, their leadership is questioned and even attacked. Upon meeting, they learn to work together to overcome or, at least, appease the will of their people, always putting their people first.
• – Both are harshly judged by a parent or parental figure for the hard choices they make that the parental figure does not agree with.  
• – They each learn more about leadership from the other.  
• – They both feel responsible for the death of someone they loved.  
• – The circumstances of their leadership means both affect the other’s fate in various ways: Lexa’s favouring of Clarke will alienate her from people and lead to her untimely death.  Clarke’s favouring of Lexa will also alienate her from her people and place her on Lexa’s throne, likely in the path of her own demise.
• – Each is betrayed by a once-trusted male figure, who sought to control their behaviour and judged them for trusting one another: Titus with Lexa and Bellamy with Clarke.  Titus and Bellamy both feel betrayed by Lexa and Clarke for their (assumptive) ‘abandoning’ of the ‘requirements’ of their leadership.  
• – Lonely and with the weight of the crowns they bear, Clarke and Lexa found their only real comfort and safety with one another.  

It’s interesting, too, how, the longer they know one another, they become more like what the other was – Clarke takes on the look/dress of Grounders, while Lexa (in Polis) adopts a more relaxed look.  

Each woman walks the steps of the other: while Lexa goes first, Clarke is clearly set to emulate.  Clarke ‘elevating herself’ is a clear line to a Commander’s ‘ascension,’ and the moment we hear Lexa speak this to Titus, we know that Clarke will be Lexa’s ‘heir.’  It will be interesting to see if there is a sort of backward parallel for Clarke now that she is alone: just as Lexa sought to bring her people into the light (enlightenment), will Clarke, now that the imminent threat of dangerous radiation is upon them, be forced to lead her people into a literal darkness (underground), to save them?  

If I was to make any speculation over what will happen in the 4th season, it would be that Skaikru – once thought by Grounders to be Mountain Men, or, like them, will have to find their own underground home in order to survive.  

A tragic cycle seems to be in place.