There are times I'm planning ahead for my character(s), especially if I'm getting close to leveling up. So what happens if I choose this spell? Should I multi-class? Oh, I think I can get a Feat now, but then I won't bump up my Strength......

Could the site and app have a Sandbox Mode feature, where I can change the level, add or take away spells or features, without affecting the original settings? Right now I make a copy of that character, fuss around with it, then delete it. 

I'm not sure I understand the request. As you mention you can already make a copy of the character, make any changes you want to see how things might be. This includes adding and removing levels, changing choices made (like ASI/Feats), changing equipment, etc. This can be used for planning the character's 'build' to have an idea of where you want to go with it, and of course remains open to change as your plans change. None of this affects your original character, so it is easy to "reset changes" by just deleting the copy. 

This is, for all intents and purposes, the exact same result as a "sandbox mode". So, I'm not understanding the request for a slightly different way to do what you can already, very easily, do? 

I don't want to have to go through the trouble of copying and then deleting. Right now if I want to see what my character will look like at level 15 I have to change that dial, but then I have to go through the trouble of putting that back and entering the correct XP, then I have to remove all the spells I changed. Oh right, I added that Feat, have to delete that too. I make the copy so I can not affect the original character. A Sandbox Mode option would just allow me to make all those changes, even going up to level 20 or beyond if possible, to get a feel for what I might want to do for the evolution of the character. At next level up should I go in this direction or that, but the original stats are safe. 

Quote from izgerry >>

I don't want to have to go through the trouble of copying and then deleting.  

 
Making a Copy: Collections > My Characters > Copy.  | Took me 5 seconds.

Deleting a Copy: Collections > My Characters > Delete > Type Delete > Took me 7 seconds.

This includes time to load and I must mention my brain is slower than it should be due to a brain condition. I therefore don't understand "the trouble of copying and deleting" when the entire sum is 11 seconds - less for most others.

If there was a sandbox mode you'd need to navigate to the character sheet, open the sidebar, click settings and/or click to enable the sandbox mode. It'd be the same or more amount of time. So, this seems like a non-issue.

Quote from izgerry >>

Right now if I want to see what my character will look like at level 15 I have to change that dial

 
I don't understand what you mean. There is no dial. 

Quote from izgerry >>

then I have to go through the trouble of putting that back and entering the correct XP, then I have to remove all the spells I changed. 

 
Adjusting the XP would be the same with or without sandbox mode - you still would need to go to the xp and change it to what you want. Even without Sandbox you can add or remove levels without needing to consider XP. 

Without sandbox you can click Manage XP to automatically and instantly set your XP to the relevant level - without needing to type the amount. You can also just use the "Level" drop down to add or remove levels, or click the X to remove the class entirely. This would not be any different in "sandbox" mode. Removing levels automatically removes any selections and spells you made for those levels.

Everything you seem to want from sandbox regarding XP and levels, is already there.

Quote from izgerry >>

Oh right, I added that Feat, have to delete that too. 

 
You can already add and remove feats as you want. You can do it as part of the level up or you can just add it manually to the sheet. You can also remove the feats. You can even remove any prerequisites so you can add any feat even if you aren't otherwise eligible. You already have total freedom to add, remove or change feats however you want, whenever you want. I can literally go to my level 4 character and change the ASI to any feat. Or remove it. Or I can just go to the sheet and 10 feats to a level 1 character, able to ignore any restrictions, if I want. Complete freedom. This is something you can already do. I therefore don't understand what the "sandbox" would do differently?

Quote from izgerry >>

I make the copy so I can not affect the original character. A Sandbox Mode option would just allow me to make all those changes, even going up to level 20 or beyond if possible, to get a feel for what I might want to do for the evolution of the character. At next level up should I go in this direction or that, but the original stats are safe. 

 
Again, can already do this?

Even without making a copy - you have a lot of "reverse the change" things. 

Let's say you were a level 3 character and got a level up, and you were considering multiclassing into Wizard. You can go into the editor add the Wizard level, select your spells and such. But after that you decide you instead want to increase your main class to get the 4th level ASI/Feat and increase your Dex by 2. You can go into editor, click the X next to wizard and now you're back to 3rd level, you can then increase your level to 4 and in the ASI choose to increase Con by 2 to add some health. All spiffy. Oh, but then you decide actually you want the Alert feat. So go back to editor and in the ASI box change it to the Alert feat - you'll lose that +2 con and the health increase it gave you putting your health back to where it was before and now have the Alert feat. 

You can do all of this, without having to worry about anything - it's all reversible with no problems. But making a copy if just better for peace of mind, in case ya forget something, while you play around and then when settled on your decisions, to make those changes to the main one. However, you don't need a copy because all your decisions can be undone.

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Every example you have provided for your sandbox idea, you can already do. Can you provide a detailed example (step by step) of something you want to do in a sandbox, that you cannot do already?