Thanks for the invite and sorry about the late reply!
Yes, reply upvoting is in the roadmap. If you take a look at the feature requests page, you will see that it's a priority item. :) Right now we are changing the way we structure and store upvote/downvote data. Once it's done, reply upvote will be introduced. I don't have an exact date, but it will probably be ready by month end.
In the rising tab, we have hot-ranking algorithm. Previously we used to have a similar algorithm for ranking comments/answers. But soon we moved to a simpler and straightforward solution i.e. rank by appreciations and upvotes. It's also worth mentioning that appreciations carry more weight as compared to upvotes. I don't think we are going to implement hot ranking and similar algorithms in any other place in near future. Right now the focus is on how we can make the platform more intelligent and suggest useful and relevant content to you.
Hope this answers your question! Feel free to ask anything else you want to know.