Commenting to answer, JB Dorman Comment, I agree with SoulLink, You are not the targeted audience on that feature. While i do agree that RPG player are the one USING the sheet, they are not the one MAKING the sheet. A proper sheet with the lik of automated calcul, modifier application and so on is a dev job. And the function of this feature is not to let anyone add sheet as they need for themselve it was at the start to allow RPG author to implement support for their game on the plateform. Which is by essence a dev job. As for the meat of this request, the system you discribe open a lot of trouble (who own the image right, how do you use it on mobile, how are we suppose to store the data in the system database... and so on) The idea seem easy but will result in very unsavory experience in my opinion (hence down vote)
Yeah, a more easily customized sheet would be extremely helpful. I also agree the current system is extremely inelegant. I know someone claimed "The solution is elegant. Your not the target audience...", but personally I think that is an extremely arrogant and objectively incorrect statement, given that it implies that the target audience for TTRPG stat sheet development is programmers instead of TTRPG players. Tangent aside I think it would be beneficial if TTRPG players were able to make the sheets as the feature intended them to, being the implied target audience, instead of walling it behind a skill that is completely separate to world building.
The solution is elegant. You're just not the target audience. I agree that it would be awesome to have a way to create sheets without knowing how to script. However your solution would no work for mobile users or at least you would have to make a separate sheet for mobile users and for desktop users.
This sounds like a really cool concept, though I have absolutely no idea how much work it would be to implement.