Been playing Monster Rancher 2 a little again, revisiting the series to solidify some of my thoughts on the game I'm working on...
So, something I've talked about a few times before is how a lot of older (mostly PS1-era) games had a certain quality of mysticism to them - when they have RNG calculations that the player can only assume, or mindbogglingly enigmatic conditions for events. Monster Rancher falls pretty neatly into this pocket.
In general, I feel like these aspects can make games fascinating and charming, but also frustrating to play. There's a lot of this that my ideas whittle out - mainly trading out the luck-based training and the hit chance in battle that could make XCOM blush for largely player skill-based & monster stat-based alternatives.
What worries me is that I think a lot of the charm of the Monster Rancher series rests in the randomness, actually. The heavy emphasis on RNG is kind of what makes it exciting in a gambler sort of way - you hold your breath on just about everything because you never know when you could fail beyond your power or snatch a roaring success from the jaws of failure.
Like, yes, I'm endlessly frustrated my 75% hit chance attack regularly misses 6 times in a row, but the enemy's 25% lands 3 times consecutively with a crit on the last one for a KO, 
but knowing that it's possible to have incredible underdog wins and massive upset losses is part of what makes the games feel so charming and alive.
And that's just it - having a game where so much of the outcome is up to numbers crunched outside your control makes it feel more like a lived world beyond the screen that you're allowed to visit than a game carefully and clinically designed for your amusement.
So anyway, that's a concern I'm facing right now. I like, don't get me wrong, I believe in my ideas here - at the worst of times, I'm sure there's a niche for the loop of monster training, but without quite so much left to chance - but I also worry that it's going to lose a lot of the weirdness & charisma that made Monster Rancher games a beloved success; I feel like, all said and done, it's courting an entirely different audience than Monster Rancher did (and a different part of my own brain, haha!)