Spline Interpolation is a technique that tries to approximate to a high degree of certainty more complex curves using simpler piecewise curves. Using it in this case would be a step backwards because the curve we would be approximating is not actually that complicated.
Diminishing returns on upvotes is an interesting idea but I don't think it would work. Even with superlinear rewards, I put all of my stake in to one account and then post comments with n+1 accounts, where n is the number of unique accounts I need to vote on to avoid such diminishing returns. The large account spreads out votes and avoids the diminishing returns.
You could also implement this is as an account to account time based diminishing return. Despite being expensive in the database to maintain, it would hurt content creators in the long term. Repeat customers are hugely important in media. YouTube, for example, puts new videos from channels you have watched in the past hoping that you will watch more of their videos. Punishing the author for creating a following will not help grow Steem.



