I'm a serious player and I'm not a fan of this idea. I prefer the extended playability , the costs , the extended planning before building or in the middle of it and more. It's sometimes more costly or less costly but in the end , I learn a lot from it and do very well in builds.
I mean I like the sound of it the sandbox idea ,until I really think about it. It does sound nice but it has the downsides.
The experimentation is what kept me playing , especially since it wasn't too easy to get quite good. It's already pretty easy enough anyways to get so many crystals and mythstones nowadays. It's not like the older patches since I played when crystals used to be way harder to get and everything else combined. I used to be a big newbie but with practise , I shine in many areas now.
A sandbox mode would as @ObiWanKenobi said kill all of that and make it almost too easy. Also sometimes the best of builds happen due to pretty harsh costs , nerfs and planning as well as asking questions. The best of players tend to also fall into this category or people aspiring to be really good.
I do use root but not to cheat. Only to save battery life and speed up my phone or slow down the phone a little bit for extended playability and keep the fast farming still there. I don't thinks truly serious player would cheat .
I mean I guess it sounds like cheating to have no zombies appearing and DQ not slowing down but it's not really. DQ physics do slow down at lower clock speed and speed up at higher which is simple and why faster phones yield faster and easier farming results , especially in packsize. This is me coming from the S3 mini which I didn't quite like when it came to DQ farming to an S6 which I've used for 1 year now.
DQ slowing down almost feels like cheating in arena or PvE but it's the way it is. I definitely prefer fast DQ way more though . Even better , fast DQ at best looking settings , though I tend to use low settings for best experience in farming.
As far as everything else goes, we already have sandbox aka dummy mode for arena and A.I modes but maybe a sandbox mode to fully test out level 100 gear and measure the DMG in love with a mode where it's single enemy or groups as dummies. Totally stole that idea from Eternium .