For modding, you'd mostly have to use the effects that exist. There are probably a few that aren't used, but you'll have to find them.
Not sure if anyone has tried this but I was wondering if anyone could make items with certain effects like an Adrenaline potion that fills the limit bar?
It's possible that this effect exists, but it's undocumented. However, you'll have to experiment with the modifiers at 0x12 and 0x13 to find it.
Another potion to reproduce the great gospel invincibility effect.
Easy enough. Just have an item grant "Peerless" status. You could have it full heal too.
"Taunt" Item which could make characters do their victory pose, hopefully they reset if they get a command or are attacked.
This would probably have to be a script action. General Counter on everyone, if hit by this item... but then the script commands necessary to do this may not exist either.
"Play dead" I just but this in for the lulz. An item that puts the character in their knocked out pose, but will regain their health and magic, while a monster doesnt attack them. But as soon as they get an order, regain canceled, so is monster ignorance.
Kinda sounds like a combination of Sleep, Death, and Megalixer. But Death overrides all statuses and removes them. Unless you can find out how to make a character stay down, this doesn't seem likely without programming the effect into the game yourself.
They're not bad ideas, I just don't think many of these would work. For now, we're all limited to effects that are known to exist.