I put some thought into what a possible moveset for Isaac/Felix could be:
jab: standard sword swings
Grab: Catch psynergy
f-tilt and u-tilt: thorn psynergy which causes thorny vines grow around Isaac's/Felix's sword when they swing it
d-tilt: a mini quake
fair and u-air: same as the tilts where they utilize the thorn psynergy
down air: would work like Link's down air, but a stalactite forms around the blade and breaks apart when it makes contact with an enemy or platform. any rock shards that fly out cause a tiny amount of damage
up-smash: has a big stalagmite come out of the ground
forward smash: Ragnarok psynergy
down-smash: punches into the ground which causes rocks to form around Isaac/Felix
neutral special: sand psynergy. Isaac/Felix hide in the ground, can move around, and after pressing B again (or after time runs out), they leap out of the ground, doing an up slash as they re-emerge.
up special: growth psynergy; if used in the air vines grow from Isaac/Felix's hands which can be used to recover on stage. If used while grouned, the vines can pop out from the ground and used as a way to trap enemies for a short time and maybe cause small damage
side special: move/force psynergy. This would work as a charge attack. a low charge would throw out the move psynergy, which would cause a little knockback and a little damage, while a longer charge would change the attack to the force psynergy, which would have more knockback and cause greater damage
down special: Earthquake. Would kinda work similarly to DK's down special
final smash for Isaac: Judgement summon
final smash for Felix: Megiddo
A difference between the two besides their final smash could maybe be that Felix has a stronger sand psynergy attack, since the sand psynergy originates from TLA, while Isaac has a stronger catch psyergy/grab, since IIRC, Catch psynergy is something that Felix can't really utilize until Isaac joins the party near the end of TLA.