Here's a list of all Materia in FF7 (for reference):
I played the demo at E3, and based on that knowledge, materia that wouldn't translate 1:1 would be...
Exit - This materia allows you to escape from battles.
Could it be reworked? Possibly. It could make the enemies stop attacking you and give you a chance to run from them.
Cover - With this equipped, you move in front of other characters to take damage for them. Activates randomly.
Could it be reworked? Not easily. They could make it more like a "sentinel" skill where you attract attention from the enemy so they attack you instead.
Enemy Away/Enemy Lure - These reduce and increase random encounters, respectively.
Could they be reworked? Yes! Could easily change how many enemies end up spawning on the map, or even prevent them from spawning at all.
Long Range - This allows you to hit full damage from the back row, as well as hit flying enemies with melee weapons.
Could it be reworked? Not easily. They could make it so that your sword swings send out energy blades or something, and Tifa's fists throw out energy beams?
Pre-Emptive - Raises the chances of getting a pre-emptive strike in a random battle.
Could it be reworked? Nope. I don't see how they could do anything useful with this, as pre-emptive attacks don't seem to be possible.
Double Cut - Allows you to strike 2x to 4x per attack.
Could it be reworked? Not really. The way the battle system works now, you're constantly attacking anyways.
Slash-All - Changes your normal attack to hit all enemies at once.
Could it be reworked? Yeah, it could be changed into an AOE skill or something similar that allows you to spam group attacks.
W-Item/Magic/Summon - Allows you to do two of these things in one turn.
Could it be reworked? Actually, yes, quite easily. Since moves take ATB charges, you could make it so that each move takes half of an ATB charge, effectively doubling your turns.
Added Cut - Adds a regular attack to whichever materia is paired with it.
Could it be reworked? Not that I can see. You're constantly attacking already, so an "extra" attack when you use an ability wouldn't make a difference.
Sneak Attack - There's a chance the materia connected to this will be used at the beginning of a battle.
Could it be reworked? I can't imagine how. Similar to pre-emptive, since battles initiate differently, the only way they could incorporate this would be to fundamentally change how it works.
As for summons, it remains to be seen how they're being incorporated, but it may be that the summons come in and attack while you are still attacking. The first summon materia you get isn't until after Midgar, so it looks like they're adding all new summons to the Midgar section to fill that gap.