I'm not sure what you mean with a "custom" time limit? Do you mean you can change it yourself to make it harder or easier? Because that doesn't really make sense to me since you could just give yourself essentially infinite time, defeating the purpose of it even having a time limit... If I'm misunderstanding please do tell me!
With that being mentioned I disagree with getting a rank like this, ranks have always been things you can buy, and only buy. I don't feel like that should change. It'd also create a lot of mess with different ranks, which would simply get confusing. A cosmetic could work, but as mentioned by
@LukaG_ this is kind of already a thing in the lobby, although I'm not opposed to more being added in the lobby!
If you did mean in the parkour game I'm more inclined to disagree however, as I feel like the points and medals are enough of a reward as it is, especially since there's already a time limit on getting those, so I'm not sure how you would even be able to fit in another time limit on top of that..