I would like to see a feature that enables selecting between 'shortest path', 'include married / in-law relatives', or 'show only blood relatives'. I frequently stumble into this problem with my (fully fictional, since I use this a writing tool) family tree where we have two people who are something like third or fourth cousins, but whose close relatives are only listed out as 'ex-partner's aunt's partner's mother' or something like that, when I would actually need to see their true blood relation - or if there are several of them.
A second cousin is still mostly fine to marry, as a singular occasion, but if two people are 'first, second, AND third cousins', as happened with one fictional (and mostly anecdotal, in this case) member of royalty, it would already begin to affect their health in serious ways, if they weren't too lucky.
Another request I would ask you to consider would be to include a possibility to calculate a person's coefficent of relationship with any individual in the tree. It's pure mathematics, and a feature frequently found in any animal breeding websites. For example, cat breed associations that have online access to the breed's registers and enable the members to try out potential couplings between individual cats to determine whether a planned-out breeding is recommendable. It's entirely possible to calculate it by hand, too, but it would be easier if it would be included in the code, especially in the case of very convoluted family trees.