Completely new abilities? Let's do this. Are you thinking defensive or offensive? I'll do all.
Gamble: Every time target creature uses its skill, it becomes -1|-2.
Ritual: Sacrifice target creature. Ghost Stallion gains +1|+1.
Galvanize: Target creature becomes -2|0 but gains Momentum.
Block: Target loses 5 HP. Anything that targets it has a 25 % chance of missing and attacks directed at it also have a 25 % chance of missing. Can target players or creatures.
Sympathy: This creature becomes -1|-1. Target creature gains +3|+3.
Frenzy: Target creature becomes +1|-1.
Tidal Wave: Target creature becomes 0|-2. Gain
.
Advent: Target creature gains +1|0 for every card summoned to the field.
Wings: Target creature loses its active ability and gains the Dive ability.
Revert: Target creature is restored to the form it was in when it was originally summoned.
Shadow Bond: Gain
. This ability can be used multiple times per turn.
Mirage: Summon a copy of the target creature. The copy has 0 health.
Some ideas are similar or are simply pulled from other ideas I found in the forums. Their costs should be scaled due to their relative power. For example, Mirage is quite powerful and should cost 3
while Frenzy is not too powerful and should cost 1
.