Ok, Diabolica has a short demo but I wanted to give Mr. Adon my thoughts about it since I think this is a solid project with potential to be a nice RPG experience.
Before reading you must know that I'm not a native english speaker so I apologise for any possible mistake you might find.

--The Beginning--
The beginning of the game is a very important part. It decides whether you will keep playing or just quit and add to the list of "tried to play and didn't like" games.
As soon as the game starts we see the main character standing there and a beast attacks you. This is a very rush start. No introduction, the character barely talks and you are already fighting a battle, not even knowing where you are and what's going on.
Since this is a demo I want to think that this is something that is going to change. At least an introductory secuence showing as something about the set.
Something I want to add is how tough that first battle was. I almost lost and I don't think I did anything wrong. The fiend just hits hard, has tons of health and can even inflict negative status. On the first 30 seconds of the game you already are in a battle that is hard and long enough to be qualified as a boss.
After that, you get the game itself. You get a little cutscene where you meet who appears to be the co-protagonist of the game. With little information of what you are supposed to do you are thrown to an epic adventure. Maybe you need to express better the characters motivations on this adventure. I guess it's okay for now but could be better.

--Plot and Characters--
We haven't seen so much about the plot to have a solid opinion about it. However, I would say it's interesting enough with some used ideas but with it's own style that makes it unique. We still have to see more about the plot but it looks like it's on the right way.
The characters are likeable enough. We get to see a couple of cutscenes that makes them less plain as it looks. There's also a diary that helps giving a lot of background information about the characters, which is very interesting to read but would help if you would have told me about the existance of this diary. It was a lucky coincidence that I realised of it.
Anyway, to sum up, I like the characters.

--Gameplay Mechanics--

I don't want to talk negatively about the gameplay just because there are some things I don't like. It does well a lot of things many RPG Maker games fail completely. The battles are neither too short nor too long and are simple without forcing you to just spam the "attack" command 3-5 times on the same enemy. The items, armors, skill and everything is well balanced. The battle mechanics are good but it does have some issues.
For example, I don't like the passive enemies idea. It's nice that you can bypass combat if you don't want to fight or you just want to travel to an specific place but the fact that you can completely ignore enemies gives you no motivation for fighting them. The only reason you have is that you think "If I don't level up I will get my **s kicked with the boss". In my opinion that's a boring way to force the player to level up. Also, the battles are repetitive after a few times. Luckily you don't need to spent hours grinding levels so it's not unbearable but it IS boring. That's how I felt playing the game.

Something cool is that there are a lot of different NPCs. There's people running, people with animals, kids, adults, old men, there's tons of variety and they even say different things when you talk with them multiple times. Also, npcs don't talk about the hero's objective. They just tell you about their life, their problems and their interests. These thing make the town really lively and realistic.
The synthesis system gives you a good motivation to talk with people, since some might teach a thing or two about some very helpful recipes. And it's fun to see NPCs living their lifes without being completely useless and pointless.

--Conclusion--

Up until now, Diabolica is a fun experience that seems to have a lot of potential. There's still a lot to see so, right now I wouldn't say it's more than a decent game. I expect this to get way farther than that and I think that Mr Adon will make it happen.
I would recommend it to any RPG lover.
Keep improving and working on this, I will be supporting you :).