You can already do Marriage > Age > Less than > 15 and things of that nature.
You can already sort of do "end of line" in the sense of Number of Sets of Parents > equals > 0 The trouble is, end of what line? So for example, if you wanted to find all of your personal end of line parents, you could do the following. Clear all colors. Color all your ancestors as red (or any color of your choice). Then do a search for Number of Sets of Parents > equals > 0 AND Color Coding > Is Red. If you wanted to make this sort of query easier in the sense of not needing color coding, you would still need a way to tell the query the end of what line.
You should in theory be able to search for Number of Spouses > equals > 0 AND Number of Children > 1 to find children with only one parent, but it doesn't work. I suspect that Number of Spouses is being interpreted by RM as the number of times the person appears in the Family Table (the number of FAMC records the person has, in terms of GEDCOM). It seems to me that Number of Spouses needs to count the number of times the person appears in the Family Table AND that the other spouse is not null. There is a lot of guesswork here, but if my guesswork is correct it's probably working as designed and the design needs some improvement.
RM supports searching for Number of Children, but it's not at all clear to me how that works. I suspect it's People Without Children rather than Couples Without Children. It's hard to test all the possible permutations of how this could work.