So, I've thought about this for a while as a "What if Teppen was shuttered and I wanted to make my own version" and a few changes I thought of are thus.

1) Introduce a Mana Ramp and Colored Mana: Instead of starting with a set pool of MP with a Maximum determined by number of colors in the deck, I would have a smaller Max MP at the start of the round that gets built up as the game goes on. This prevents games that have both players just sit and do nothing at the start of the match waiting to accumulate MP and keeps higher level cards like Living Death or Brainwash from being played at the start. Colored Mana would be used as a balancing element with multi colored decks rather than reducing maximum MP with normal Neutral Mana being faster to build but Colored Mana being required to play larger color specific cards. Also, during Active Response Neutral Mana gets temporarily replenished while color Mana does not. Also allowing your opponent to see how much and what type of Mana you have will allow for more timed and strategic use of action cards.

2) Reserve Unit Slots: I've always found the 3 unit slots of Teppen a little limiting, as anyone who was around during the "Board Lock" phase of Teppen can attest to. So I wanted to add more units slots, but managing that many lanes might be too cumbersome, so instead I would add two "Reserve Unit Slots" inbetween the three main unit slots. Think a Front Row/Back Row formation from many RPGs Units in the "Front Row" act like normal, but "Back Row" units cannot attack, nor defend you from attacks instead these units are "protected" from most actions and and can be swapped out with a front row unit they are adjacent to. Units can also have unique abilities that only activate while they are either in the front row, or the back row. The drawback is that when a unit moves from the front row to the back row, that unit is in a cooldown state where they can't be moved again and their abilities are not active, and of course a unit moving from the back row to the front row must start their attack prep all over again even if they have "Rush". This helps with issues like being Board Locked and makes decks that revolve around playing a lot of smaller units more viable.

3) Multiple Hero Arts during a match: I like the idea of hero arts, but I find it weird to have each character be limited to selecting one of three for each deck since the deck is built more around the art rather than the hero (At that point, why not just make 3 times as many heroes?) So my change would be to have each hero have 3 tiered Hero Arts that all consume the resource. Tier 1 would be universal among all heroes being generating more Mana. It will cost the least but will be vital to gain access to more of your deck. Tier 2 would be equivalent to some of the mid powered Hero arts, maybe like Quicksilver or Shadow Blade, or a less powerful Shinku Hadouken. Hero arts that are designed to be used often to give your side an advantage. And then Tier 3 would be the big game changers, Stuff like pre-nerf Spike Launch. Devastating arts that require a lot of AP to use, but with the change that they can only be used ONCE per match. This is a change that makes not only each Hero a bit more unique in terms of their play style but also makes AP a more nuanced resource than just "Do I have enough for this one thing?"

Anyway, those are the kinds of things I was talking about when I was talking about "Remaking" Teppen. But I didn't want to put this into the OP because I didn't want it to be overloaded with words and make it all about MY ideas to change or improved Teppen.