Marth
3111
1113
1133
1121
3133
3131
Only 1 preset involving Crescent Slash?
This is Marth/Lucina's best custom, in my option.
Is a true combo at low percents from a grab.
Insanely fast, basically a teleport with a sweet spot.
Relatively safe and can be used to go on Battlefield/Smashville platforms diagonally.
Great KO potential. Can even be used at the edge of the stage and come back on free fall.
1121 is a great preset, but I would think that 3121 would have been great as well. Maximum mobility options, especially in neutral and combo potentional, and it can help their recovery horizontally in combination.
Stage spiking is still very usable also, just obviously not as deep as Dolphin Slash.
After extensive tests/games, I don't feel that Iai Counter is that good.
The window is very tiny and the counter is very interruptable. Examples:
ZSS's uptilt will beat this counter on the second hit.
You will not even be able to use this move with PK Fire unless it's on the last hitbox.
Little Mac will armor frame this because it's too fast.
Basically, any multihit move has a great chance of beating this move, easily.
The positioning is also an annoyance since more often than not, you're facing the opponent so having the opponent fly behind you isn't ideal.
The move is flashy and really awesome, you can get some potentially great early kills or edgeguards with it, but I think that regular Counter is superior.
Also, as far as their Easy Dancing Blade, it might be usable because it can combo a lot easier. The hitstun is better due to having more window, and it does less knockback. At least with Lucina, the last hit isn't a great kill move at all anyway.
tl;dr - Marth/Lucina should have 1121(currently does) and 3121 for more faster mobility options. Less Iai Counter.