I'd be interested in this, provided it handled a few things that I'd like to see.
1. Decent draw distance. In most of these games that I've played in the past, the draw distance has always been... very minimal. We're talking 10 squares at most. I'd like to see the option to have viewing of at least twice that distance. If you could get it to work up to 20 squares away, you could build some pretty impressive sights. Which, leads me to 2.
2. I'd like to see it rendered a little more "3D". While moving on a 2D plane is fine in a 3D environment, it lacks a certain... "awe". If you could get balconies within dungeons to view "down" or even "up", I'd prefer that sort of system. Having such a thing would allow a certain level of "environmental storytelling" as well as other options like possible platforming that we see in games like Ultima Underworld. Yes, I'd even settle for a limitation where to achieve the effect, you have "transparent" ceilings and walls to give the effect of seeing above and below. Just so long as you can see other Z-Levels.
3. I'd be curious if the mode could be used for the entirety of gameplay. From walking around town, to exploring dungeons. I think such a mode would make towns feel more "lived in" and be a bit more immersive.
4. Smooth animations would be a "must". From "turning" to "walking".
5. Ability to customize the assets, or even insert assets into the environment. Chests, paintings, water, lava, switches, bosses, teleports, whatever. Blank walls, or similar looking walls tend to pull players out of the experience and tend to wreck the immersion. I wouldn't mind having to do these myself, I'd just like the option to do it myself, instead of having it be a limitation.
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All that being said, I'm always interested in new game makers that this website produces. If I can imagine a game I want to make with them, I generally just purchase them regardless of the price. I like the "simplistic style" that the company seems to work with (IE: Easy to understand, easy to code a basic game, mastery of it is more difficult and time consuming).
I'd have coded a 3D RPG in Unity if the program felt any sort of "user friendly" to me and stock assets weren't all "mainstream". But, a 3D dungeon crawling experience in a regular RPG Maker game would be kind of cool or neat in my opinion. Especially if you can tinker with it as much as you can the Vanilla products.
Heck, depending on how well such a feature is made and supported, I'd honestly be willing to drop $120 USD on it. Especially if you could recreate something close to the old FPSRPG styles. I bet I could find a game I'd want to build with it.
Now... if only we could get an Isometric RPG Maker....