I was playing around with unique moveset with Isaac in my own head today. Specific moves have all been discussed, but I came up with an interesting gimmick, although it may sound similar to those mentioned before it.
These little guys....
The djinn are the highlight of Golden Sun's battle system. The strategy to Golden Sun's battle system is that when djinn are set to Isaac (and the others members of your party) they increase that character's stats and grant access to different psynergy (magic) attacks. Your djinn can directly assist you in battle by attacking your foe with a more powerful strike, but after attacking they disappear for a bit. When djinn are gone, your character becomes weaker and their psynergy options become more limited, and they can't use that particular djinni to attack until it resets to their character (based on a turn timer). Essentially, it's the art of achieving the balance of immediate all-out attacking power vs stat increases and magic options at the right moment.
I think if this general idea was incorporated into Isaac's moveset it would give him a very unique gimmick, (as if swordsmanship + earth magic wasn't unique enough), something along the lines of Little Mac's power meter, Olimar's Pikmin dependent moves or Lucario's aura system; djinn augmentation.
In my idea, Isaac's A moves would be what you'd expect, generic sword slashes and kicks and such, but with his smash attacks, you would have the option of using an earth djinni for a VERY powerful follow up hit to augment either your Up Smash, Forward Smash or Down Smash. Think of this follow up attack as being similar to Link's second smash A backhand slash or Metaknight's second tilt A. For example, say Isaac's Down Smash makes him stab into the ground with his sword, causing rocks to appear to either side of him, but follow this attack with an intimidate press of A again, and your djinni appears above Isaac's head, slams down into the ground and causes the aforementioned rocks to rise out of the ground and shoot off to either side of Isaac for massive attack power AND with additional range!
However, if you do use your djinni, it's gone for a limited time. As a result, you can't use it again to augment any of your smash moves again until it resets to Isaac. Furthermore, the power and range of your psyenergy (special) moves decreases, as does the range of your recovery Up B move. Like in GS, the djinn resets based on a timer, say following a 20 second recharge. This prevents these powerful moves from being spammed and comes with a very real price. After the 20 second timer passes, your djinn automatically returns above Isaac's head in descends into his body, along with that classic Golden Sun chime to let you know "Flint is set to Isaac!"
Now, this would be perceived as a very Golden Sun appropriate djinn system, and could make Isaac a very strategic, RPG-like, high risk, high reward fighter in Smash Bros,
How will you approach your foe? Do you conserve your djinni and use you normal sword attacks and psyenergy to attack Bowser? Or do you use your djinni now to try to attempt to KO Bowser immediately 50%? Be careful though, he may very well dodge and you'll have wasted your djinn for nothing! And with out your djinn to help you, Isaac's Up B's teleport recovery range is halved and your projectile B gaia move doesn't even cause Bowser to flinch! What to do?!
What do you say? Interesting idea?