Would this be a good idea ?

What are the pros and cons ?

I think with the higher threshold this indeed might be an interesting option.

It would stop psy power spamming and would also make sense as it can be exhausting to control the powers of the warp without drowning in the waves of chaos.

What do you think ?

I don't think it makes a whole lot of sense. If you use one psychic power each turn, you are gaining Fatigue at the same rate as someone who is suffering from Blood Loss!

With that said, I think there needs to be some limiting factor on the use of psychic powers. Personally, I'd like to see the Fettered power level removed or altered so that there's still some chance of causing Psychic Phenomena. Maybe they still caused psychic phenomena on doubles, but you'd get -10 to the result of your roll on the phenomena table? There are a couple talents that can mitigate Perils of the Warp in dire circumstances (Warp Lock, etc), and the AAT background helps with damage control when you do cause psychic phenomena, so psykers can still control their Perils of the Warp rolls to some degree.

Going by the fluff for psykers, there tends to be three things that happens during psychic powers, the psyker perils (the game already copes with this to a certain extent), the psyker becomes tired (fatigue seems the right thing to do here) or the psyker has to take time drawing on the warp to power the effect.

Personally I'd be happy with the fatigue effect, but it could mix things up depending on the power, for example pyromancy powers take time (one round to draw flames, one round to fire them), biomancy causes fatigue (toll on the body), telepathy causes insanity/corruption (touching unhinged minds), divination causes stronger perils (paradox of seeing the future).

Anything to make psyker power work better within the system gets the thumbs up from me, be it fatigue or lossing fettered/+5 to rolls for PR.

I don't think it makes a whole lot of sense. If you use one psychic power each turn, you are gaining Fatigue at the same rate as someone who is suffering from Blood Loss!

Honestly, that's the point. With the provision that "minor" powers (thought sending, using telekinesis to move very small objects, using pyrokinesis to light your lho stick) wouldn't cost fatigue, I think it would be a good way to impose a simple limit on the use of psychic powers. As Naviward points out, it fits with the background.

Maybe give each psychic power a power level 1-10. If your psy rating is equal or under that number, then you must take a toughness test or gain fatigue when you use the power. Psy rating is over, then you don't need to test. If Power Rating is double, the test is -10.

So powerful psykers can more freely cast powers, but still test on big ones. Medium and lower psykers need to be careful with their powers.

E.G

Assail would be power 3

Objuration Mechanicum would be 5

Gate of Infinity would be power 8-9

Psy Rating 4 Psyker could freely cast assail, but would need to test toughness after Objuration, and -10 Toughness after casting Gate.

However, more importantly first we need GOOD psychic powers

Felenis, that is a very interesting idea.

Personally I really miss the minor powers.

They really made it feel like your character could interact with the world through the warp.

As Psykers stand right now, it all feels a bit lackluster.

Yes, minor powers were great, but rather wacky and actually sort of broken good.

I'd like an expanded version of Only Wars cantrips, as well as more noncombatant powers

Just here to say that the current state of psychic powers make me a sad psychic panda

What if casting powers at the Fettered level inflicted some amount of Fatigue on the psyker (something significant, like maybe 1d5?) instead of manifesting Psychic Phenomena when they rolled doubles? This would protect psykers from the potentially disastrous social consequences of (for example) summoning a Plaguebearer at the governor's birthday banquet, but would still punish them for reckless overuse of their powers. This Fatigue represents the effort and focus required to contain the surging Warp energies. Players make psykers because using psychic powers is fun, and a drawback that is too harsh can be extremely limiting and unfun.

Felenis, your idea sounds great but would require a significant rework of the core psychic mechanics to function.

I don't see how. Just add a number beside each power, boom, done.

It doesn't change any of the mechanics, just builds a layer on top

Would this be a good idea ?

What are the pros and cons ?

I think with the higher threshold this indeed might be an interesting option.

It would stop psy power spamming and would also make sense as it can be exhausting to control the powers of the warp without drowning in the waves of chaos.

What do you think ?

Kinda reminds me of good old Shadowrun, except I like those rules better:

First you cast your spell (here: Activate your psychic power).

Then you resist Drain (here: roll to limit fatigue loss).

And ofcourse the presence of the spell/magic pool (depending on edition) made for a nice little bit of tacticality as well.

Adds a dice roll, but I can live with that.

I've mentioned before that I'd like to see the psychic phenomena table toned down a bit and in turn have psychic powers ALWAYS roll on the table. Make it a 10% chance of having no manifestation for Fettered powers and for unfettered powers that 10% becomes the chance of rolling for perils. I'd like for psychic powers to have something to differentiate them from D&D magic, and I think that emphasizing the weird effects they have on reality would help that quite a bit.

If psychic powers could be brought in line with the rest of the stuff in the system (probably slightly more powerful, because mundane skills don't risk you getting swallowed by a warp rift on a semi-regular basis), we wouldn't have to think about slapping penalties on psykers trying to use their character sheet to the best of their ability.

Once more, I'm fairly confident the old beta's psychic system could work with current stat values if we changed all PR-derived roll modifiers from +10/-10 to +5/-5. It was probably the only change in that godawful, ill-conceived system that was actually an indisputable change for the better.

Fatigue is a good idea. We just need to make it make sense so you don't get it for small stuff.

It seems to me that the best way to encourage psykers to use their power Unfettered is to just increase the difficulty of Focus Power tests across the board.