Isaac can:
* Create earthquakes (or "Quakes"). Quake Sphere is the strongest version of this attack. Three columns appear under the enemies and they are thrown up and down by the quake.
* Make vines and plants grow (using a psynergy called "Growth") He also has a stronger version called "Wild Growth". Think "male poison ivy".
* Can cause a barrage of rocks to fall on the enemy. (The strongest version of this is called "Avalanche")
* Somewhat similarly to Growth, Isaac can cause thorns to grow up from the ground. The strongest version of this is called "Nettle".
* Create earth spires to launch at the enemy. "Stone Spire" is the most lethal version of this attack.
* Can "attack with the earths might" using a move called "Gaia". The strongest version of this move is called "Grand Gaia".
* Isaac can "attack with a bamboo weapon. From the image it appears that he can summon bamboo shoots out of the ground. This move is called "Punji". The strongest variant of this is called "Punji Strike". I think this is an interesting option that doesn't get mentioned very often.
* Isaac can summon demons (yes, you heard that!) using the move "Demon Night". There is a stronger variant of this called "Thorny Grave". Generally I wouldn't think this would suit Isaac in smash but on the other hand it might be kinda cool to have a curveball like this being part of his moveset.
* Isaac can "crush a foes skull" (yep, he's hardcore) by using one of the "Splitter" attacks. "Helm Splitter" or "Skull Splitter". The latter being the stronger of the two.
* Isaac can use his psynergy to summon a sword. Ragnarok is the one people are most familiar with but there's a stronger variant called "Odyssey". A vote for Isaac is a vote not just for one sword but also a giant freaking spectral sword. It's two for the price of one.
* Isaac can summon a giant psynergy hand (These next few abilities are known as "Utility Psynergy") which he can use in various ways. "Catch" is among the most well known and he uses it to grab various objects in the overworld. It doesn't take much imagination to know where this could factor into his Smash moveset.
* Isaac can use the psynergy "Move" in order to push various objects. This was the ability he used in his Brawl appearance as an assist.
* Isaac can summon two giant psynergy hands in order to lift and carry light objects. This is significant for a few reasons. It shows that Isaac is not just limited to summoning one of these hands at a time like he does in his appearance in Brawl as an assist trophy. We could also assume that Isaac could potentially use these hands to lift him up short distances or throw enemies off the stage.
* Isaac can use the "Scoop" psynergy to dig in soft ground. He could potentially use it as a sand attack in Smash Bros. Although I don't think it's one of his more interesting abilities.
* Isaac can use the "Pound" psynergy to summon a giant hand to pound an object into the ground. Think about how satisfying it would be to be able to summon a giant fist in Smash Bros to slam down on your opponents head.
* Really Isaac's hand based psynergy has a lot of potential. On one hand there's what we know he can do with them in the games, but on the other hand there's what we know hands can do in general. He could use them to slap, punch, poke,
give the finger or do any manner of fun stuff if Sakurai was feeling extra creative. For example he could summon a giant hand to give a thumbs up as part of a taunt or victory screen. How about if he loses a battle he summons two giant hands to clap for him?
* Isaac can use the "Halt" psynergy to cause moving objects to stop moving instantly. This could be a paralyzing attack although this wouldn't be something unique to Isaac. It is an option however.
* Isaac can also "Hover" short distances and "Teleport", although these aren't particularly unique to Isaac. However he is capable of doing it.
* Isaac can use Djinn to attack (each have a different effect in battle) or when set change his class and allow him to use powerful Summons. There are four kinds of Djinn based off the four psynergy types. Clockwise in the above picture: Venus, Mars, Jupiter and Mercury. Look at those cute faces.
* If Isaac is wielding the Sol Blade there is a chance that using his default attack will cause a weapon unleash called "Meggido" to occur. It's pretty much slamming a giant flaming meteor into your opponents face. Take that Mario! This could be an option for his Final Smash or they could create a more toned down version as part of his regular moveset.
* Isaac can, as mentioned above summon powerful summons. This would likely be where Isaac would get his Final Smash. "Judgement" in the above picture is the most well known summon and would be a prime candidate for Isaac's Final Smash. "Judgement" flies high up into the sky and shoots a beam of energy out of a Lion head cannon (Hello? Can I get an "Awesome!"?) towards the ground which then explodes into a ball of energy wiping out everything in it's path.
====Beyond this point is where I think the attacks become less appropriate for Isaac's moveset in Smash====
* If Isaac has the tamer class he can actually summon various animals and monsters to help him fight. He can summon a Wild Wolf, an Orc, a Dinox (it's like a dinosaur), a
, an Emu, a Harpy, a Gryphon, a Minotaur, a Roc, a Grand Golem, a Living Armor and a Ghost Soldier. Personally I don't feel this move would be significant enough to add to his moveset, but he can certainly do it. I feel like something like this would eat too much into the earth side of his moveset. There's enough here for an entire moveset in and of itself!
* I'm not too sure about this one but if Isaac has the "Pierrot" class in The Lose Age he gains a deck of cards that he can use to attack. Can somebody else clarify on this? However most of the attacks used through the deck are Fire and Wind based. So it wouldn't really be appropriate for Isaac. There are some Earth aligned moves such as "Sabre Dance" though, which is an attack of multiple dancing blades. Even MORE swords!
* Isaac can use an attack called "Annihilation" although it is a one hit KO move and would not really be appropriate. However Isaac CAN do it. It would probably make Isaac a great counter to Diddy Kong though. (and everyone else in the game)
* Isaac is capable of healing himself. It probably won't end up being part of his Smash moveset, but he can still do it if necessary. "Cure" is the weakest up to "Potent Cure" which is the strongest. He can also "Revive" allies, which isn't appropriate for Smash so I'm just slipping it in here.
* Isaac can also call upon the undead using a group of psynergy called....."Call" (so I suppose you could say with the right class he can be a necromancer) He can call a Zombie, a Demon and the Dullahan. Again I'm not sure if these attacks would really suit Isaac, but he can do it.
* He can also use Mars (Fire), Jupiter (Wind) and Mercury (Water) psynergy too but that's not what really makes him unique. (although there are definitely some really cool unique moves among the other elements.
* Lastly this is something only Felix is capable of but if Sakurai felt like pulling a Ness with Isaac he could give Isaac the ability to use "Sand". It's an earth psynergy that allowed Felix to turn into sand and use it to navigate the overworld. We could have a Smash fighter capable of actually turning into sand and moving around the stage. Think about that for a second.