I've been grumbling internally about the lack of aspirations in this game. I've got a list of about 38 aspirations and counting to make and in noodling in the Sims 4 Mod Constructor it became clear I need to write these things out more fully before doing the creation. The way the tutorial I'm following works you're creating custom places for your aspirations to go. I was kind of hoping to wedge mine in under the existing places and throw the existing Bonus and Reward Traits at them rather than having to create a lot of them.

I'm currently working on "Become a Hall of Famer" (Max the Athletic career's sports branch) and I was thinking that the stages would work something like:

1. Reach level 3 fitness - Jog for 3 hours - Do push ups 5 times - Work out at a gym
2. Become an adult - Get a job in the Athletics Career - Reach level 1 Charisma
3. Enter the Athletics Branch - Be Energized for 12 hours - Push the Limits 5 times while working out
4. Become a Hall of Famer - Master Fitness Skill - Mentor Sims for 5 hours

I'd like to put this in the Athletic category with the High Metabolism trait. I'm not sure about the reward trait. I was thinking the reward trait would be like "Household Name" and read "Everyone in Sim Nation knows {sim} name. Is it any wonder {sim} feels more confident?" Then they'd get the confident emotion more and act like someone who has the "self-assured" trait. I would also give that to any career aspiration with "celebrity" in it like "Celebrity Chef" or "Celebrity Mixologist."

Naturally, the next one would be Become Mr/Ms Solar System and be pretty much the same, but you'd choose the Body Builder branch and they'd get the same traits and similar flavor text. Ideally, I'd like to cover the rest of the Base Game careers and then go into the careers introduced in the various expansions and packs.

TL;DR is it possible to use existing game categories and traits with new aspirations? Does this sound like something other people would enjoy? Am I completely off my rocker?

You can use existing categories however you will need to use the reward trait for that category. So for athletic the reward trait would be long lived. If you wanted to use a different reward trait than the one used for an existing category, you would need to add a new category which isn’t hard.