So, I've only played the demo (which is a fair bit, goes up to Full Metal Hugger) and I have a lot to say as someone who's played the original Trials of Mana (AKA Seiken Densetsu 3).
Long story short it's a completely different game than the original, and I strongly suggest anyone interested to try the original out (it's part of the Collection of Mana on Switch, came out in the west for the first time not too long before the remake). While the plot's the same the gameplay couldn't be any more different.
As a bit of background, The Mana series was Square's first foray into Action RPGs. Their approach to the genre is different from most companies- mostly because they were some of the first Action RPGs ever. Since they predate Tales of Phantasia, the game to introduce combos and beat 'em up/fighting game elements to Action RPGs, before that point most games in the genre took after Zelda and Ys (a game that's older than Zelda IIRC).
Basically, Mana was more of a "Real Time RPG" than an Action game with RPG elements like most people would expect. Or you could say it's half Action half RPG, rather than a mix.
So for the original Trials of Mana (which I'll refer to by the shorthand Trials SNES) it was a top down action game. There were no combos whatsoever- it kind of translated "turn based" to real time. Each attack you do has a short cooldown, during which you're free to move, reposition and dodge. Once it's done you may attack again. Might sound cumbersome, but it was heavily improved off the first two games (Final Fantasy Adventure was the first, and Secret of Mana the second) and honestly feels really smooth. Enemy and boss design is built around it, they don't exactly have their own combos, and you can knock them down and away with different attacks.
Difference 1: The remake does not use this system at all. There's no cooldown system and you can freely cancel out of attacks with a dedicated dodge mechanic (original had none, just movement. Instead you have basic combo strings with little to no endlag.
Even though I love, love fast paced action games and would hate for things to slow down, I prefer the cooldown system. It's just more unique, there's not really anything else like it, it makes a fun rhythm when you get it down... and the remake's combo system doesn't really feel anything special. Not very weighty.
In addition to that there's a meter that fills up as you land hits, and when full you can unleash a combat art.
There's also the Ring Menu which you can use to use items and cast spells at any point.
Enemies of course have their own spells and arts...
Difference 2: Enemy Arts and spells function completely differently now. In the original, enemy arts and spells would freeze time and are guaranteed to hit their target. In the remake, there is no time freeze and instead there's a red aoe indicator letting you know where the attack will hit to dodge it.
This is the real point of contention. A casual player might say the remake's better because you can finally dodge attacks instead of being forced to take a hit. But we're not casual players, are we? We can look at the design of things with more nuance.
The truth is it all goes back to how they designed the mix of Action and RPG. As I said it was *half* action and *half* RPG. The way the Action parts work is obvious, you dodge attacks, aim your own, use the right moves at the right time... but the RPG part, that's not just the stats and levels. That extends to the combat design. Think to turn based RPGs- if an enemy attacks you, you *will* get hit. There's no option to dodge it. Maybe a random chance to, but it's always so low you feel lucky when it happens. You go into fights knowing you'll take a few hits.
In an RPG, your HP is not a "mistake meter", it is a resource. Manage it by knowing when to heal and how to minimize how much you take.
That's the design Square took into the Mana games, including Trials SNES. Enemies have many real time attacks you can dodge for the action game, but also guaranteed attacks you *have* to take. You're expected to manage your HP like an RPG. The way this plays out is clever, the better you are at the Action part (by dodging attacks and killing efficiently), the more leeway you have into how your HP is managed for the guaranteed damage you'll take.
This is removed entirely by the remake. Now every attack is dodgeable. Purely from an action game lens this might be fine, but I lament the loss of a unique mix of Action and RPG...
It also tips the balance in the player's favor. From the little I've played, I've gleamed that even though enemy arts and spells have been changed to be dodgeable, the player's are still time freeze moves that are guaranteed to hit. In the original game, even if you didn't realize the point of the design was to make you manage your HP, it still felt balanced that the player had access to the same guarantee hit moves the enemy does. Now it's only the player that has it, tipping the balance a bit. In fact... in the original, the player's level 1 art was NOT a guaranteed hit, playing out in real time, and only later level arts had the time freeze. In the remake, even the level 1 art is like that. They knew what they were doing.
And then there's the obvious differences that come with making a top down game into 3D...
Long story short, the remake of Trials of Mana is less unique than the original, which was an incomparable game. The changes just make it more in line with standard Action RPGs.
However I'm fine with this change. When Square remade Secret of Mana and kept that game exactly as unique as the original was, many people derided it (and acted as if it was somehow worse than the original when it really was the same game, just in 3D and more plot). A sad truth is that modern audiences may not be all that open to an action RPG that lacks combos and forces you to take damage, regardless of how well designed around those mechanics it is. And Trials of Mana SNES came out in the west, for the first time, less than a year before the remake, so even if we ignore the above reason, they're wise to make some change so it doesn't feel like the same game twice. At the end of the day, the remake is intentionally a game that will not replace the original, since they put a lot of work into giving the west the original as well, and both can be experienced by modern players to judge.
But there's one change I find unforgiveable...
Difference 3: There's no multiplayer. Secret of Mana and Trials of Mana were both co-op RPGs. Secret's remake retained the co-op aspect. The remake of Trials, for whatever reason, ditched this aspect. Co-op RPGs are hard to come by so it's sad that the remake didn't grace us with another one.
That's enough about the differences though. Here's what I can say about the base mechanics.
As stated before, enemy attacks are all dodgeable, there's a dedicated dodge mechanic, arts and spells have aoe indicators for the enemy but are guaranteed damage for you, and combos are fast with no endlag.
There's also a "soft lock" mechanic that causes your attacks and dodges to go towards the nearest enemy when you're not pressing the control stick. This is most likely just to make it easier to hit enemies in full 3D, but this aspect is the thing that got me interested in playing the remake more even though I know I'll like the original, because I like to do "no left stick" challenge runs in games where I know it's possible, and boy is it possible here. Probably won't matter to a normal player though. In fact, I did a no movement of full metal hugger in the demo...
Movesets are definitely expanded to accommodate 3D. There's running attacks, anti-airs, charge attacks and more abilities to unlock as you go through and reclass. You can also switch control of your character on the fly, so people who like to control an entire party have something to look forward to.
This is all things I've gleamed from the demo alone. I'll give the full game a deep dive when I finish the other RPGs I'm currently playing, lol.