Right we know why they're doing that, but if they were one and done and high mp costs and weren't limited by battle arena like they're making it now, I feel it'd make summons cooler than "Just spam summons when you're in a boss fight arena" like it is currently. Or worse, fill up the summon gauge by fighting weak mobs before you go into a boss fight and just use them there. OR EVEN WORSE not being able to fill it up or prep summons in any way minus in a boss fight. Which then punishes you for doing well, much like FFXV did, when summons should be literally "big spells."
In terms of ATB yeah we could have them add Deathblow and Steal there, but having it cost ATB means they'd have to get buffed considerably to make them worth it versus just using magic materia or limit breaks. Even using KH as a comparison point to make this system viable across the 5 characters present is going to require some animation reuse for sure. But even so those things will be incremental and tiny changes to the grand scheme of things.
Making Materia, a system that was previously a series of if/then statements into now just "buff everything for big numbers" is what I'm wary of. And if counter does exist I don't know how viable it would be given the current system of dodges and mostly one on one combat minus a few mobs here and there that might throw 1-3 enemies at you. The action system changes how all of these materia function, and several materia are downright pointless if you're limited on space and can use that slot for buffing yourself, using magic, or just enabling more damage.
The ATB system, if it remains as limited as it is currently, is going to be almost solely focused around damage dealing due to only having 1-2 charges. Even in the bigger boss fights where they weren't using it as much you have only so many times you can use spells before you run out of MP, meaning majority of the rest of it is going to be auto attacking and LBs.
Again, it's all theoretical based on what limited footage we've seen, but I don't have high hopes if the elemental materia is anything to go off of. We're most likely looking at incremental stat increases or simple mechanics to fit the simple fight system, due to the emphasis on dodging and action based combat versus having to have sort of "role" based combat before. I'm happy to be wrong though.
/shrug honestly, I don't feel much of a way about the summons. They're very much in line with what the more recent Final Fantasies did with them, and are basically another limit break given the meter. You're not punished for doing well, though; the meter builds up with every attack you do. I doubt they'd limit it to just boss fights (they didn't in other games), so it's pretty much a choice of if you want to do it on trash mobs or bosses. I kinda think it suits the idea of them as summoning something to fight alongside you, but I do understand why it might not be your favorite.
Deathblow and such could also change the non-ATB battle commands, or add new commands that don't necessarily require ATB bars for use. We only see Square, Triangle, and Circle used in the battle menu, so unless they reserve X solely for interacting with objects in the game world even during battle, command materia could add an X command (or replace Square or Triangle, even). For all we know, they could just add a new command to the Circle menu that may or may not use ATB bars.
I'm not quite sure how you're getting if/then statements out of materia, though. It wasn't Gambits or anything; it added extra stats and commands that could be modified by a few other materia. Like, linking the All materia to Bolt isn't if/then; it's just multi-target Bolt. Counter could easily be something where, if you block an attack properly, Cloud slashes with his sword after the block, or it could reduce the delay between blocking and attacking. Kingdom Hearts has it, after all.
Having seen mostly boss battles, I don't think we can safely say it's mostly one-on-one combat. Hell, didn't the combat demo show a fight against 3 Shin-Ra grunts at once? Enemy groups in the original game were comprised of 1-5 baddies, but the most common configurations I see are 3-4 enemies. I'd be surprised if the game keeps you only at 2 ATB bars, too. XIII's prerelease materials only showed the early fights with limited ATB bars, but the game granted you more as you progressed.
As far as the elemental materia function, it looks like instead of converting all your damage to an element, it adds additional damage instead. From the perspective of how the original game was, I can see how that might be disappointing, but it's not quite clear if it's normal damage + a percentage of elemental damage on top of that or if the percentage replaces part of the normal damage. Honestly, I kind of like the idea of it being percentage-based, because it means that if I encounter a new enemy that absorbs Thunder, I won't have to worry about whittling it down only to have a Thunder/Elemental link restore half the enemy's health; instead, it's a small heal.
