Looking to convert spells into invocations, looking for advice.

[frogglesmash]

In a campaign I plan on running sometime in the I want to have a magic system that is similar to how adept magic works in Skulduggery Pleasant (for those who are unfamiliar: adepts in Skulduggery Pleasant are magic users who know only one or two magical "tricks" i.e. breaking bones with a touch, Traveling through solid ground etc). In order to emulate this I plan on using a slightly modified warlock chassis and then just letting the players choose level appropriate spells from any list in place of invocations (Least=1,2 Lesser=3,4 Greater=5,6 Dark =7,8,9).

I realize that going through all the spells d&d and deciding whether or not they would be appropriate for this sort of usage would unreasonable, so I instead plan on having my players run all their spell choices past me and adjudicating them on a case by case basis. My question is what kind of things should I look out for when I'm making these decisions, what kind of things would make a spell unduly powerful if it were available as an at will ability?

For example:

Most spells that create permanent objects/creatures should have a duration, or not be allowed.

Save or dies should either have their availability postponed or be disallowed entirely.

[Nifft]

Spells with an ongoing effect (wall of ice, charm, summon swarm) can only have one such effect active at a time. You can use the spell while the previous casting is still active; doing so immediately ends the previous effect.

Spells with a conditional effect (suggestion) can only have a limited number of concurrent active effects. This limit is often your Charisma bonus.

Spells that give you a perfect disguise (spider form, aspect of the wolf) always turn you into the exact same creature. Once someone knows "your" spider or wolf, they can recognize you in that form.

[Zakerst]

An exception would be spells that create items and or permanent effects that have a cost (see the dead walk invocation for an example of what I'm talking about).
Part of the trouble with your specific creature example is that it makes the more powerful and versatile transmutation abilities all the more desirable, or otherwise may make the character nearly useless (e.g. are trap choices). In this case polymorph and alter self are the spells in question, very powerful but not in need of such a heavy handed nerf imo.
Be aware that not all spells with lasting effects are "permanent" some are "instantaneous".
While SoD spells likely should be avoided SoL and NSJL (save or die and save just lose) spells should also be examined carefully. That said I do not advocate removing them entirely as doing so is kinda unfun. After all who doesn't want baleful polymorph at will to turn people into frogs. (A possible fix would be to add a clause to SoDs and SoLs that if you make the save you cannot be affected again for 24 hours)
Please also be aware that these may make the pseudo metamagic feats much more valuable (empower SLA, quicken SLA ect).
One spell that springs to mind as kinda excessively useful is avasculate, cutting HP in half on a save can be really powerful, that said if you're ok with people running super fang in Pokemon it's the same thing.
Another thought that came to me as I was typing this: "if any of them take a healing spell they will all always have full HP for an encounter" not the biggest problem but, something to keep in mind. On the positive side this could help cut down on the 5 minute adventuring day.
Shivering touch is also really really good at will spell.

[frogglesmash]

Part of the trouble with your specific creature example is that it makes the more powerful and versatile transmutation abilities all the more desirable, or otherwise may make the character nearly useless (e.g. are trap choices). In this case polymorph and alter self are the spells in question, very powerful but not in need of such a heavy handed nerf imo.

I think nifft was trying to say that the appearance of any form you chose with polymorph, shapechange etc is the same each time you pick that specific form, not that the character should be restricted to only one form. I don't think this is a change that makes much sense since the main benefit of higher level polymorph spells is the abilities the chosen forms grant you, whereas the ability to disguise yourself freely is arguably a major feature of lower level polymorph spells.

[Nifft]

I think nifft was trying to say that the appearance of any form you chose with polymorph, shapechange etc is the same each time you pick that specific form, not that the character should be restricted to only one form. I don't think this is a change that makes much sense since the main benefit of higher level polymorph spells is the abilities the chosen forms grant you, whereas the ability to disguise yourself freely is arguably a major feature of lower level polymorph spells.

That's right -- well, unless you're converting lower-level spells, like the new [Polymorph] subschool spells, which might be "only one form" as a specific power limitation.

The arguable top 9th level spell (shapechange) might not make a good Invocation.

For alter self, maybe something like... when you choose this Invocation, pick 3 specific forms. You can assume any of these forms, or your normal form, as a Standard action.

For aspect of the wolf, maybe do something like the spider form invocation, and get better wolf forms as you level up:
- Level 1+: Wolf
- Level 6+: Dire Wolf
- Level 11+ (pick one): Winter Wolf, Yeth Hound, Nethersight Mastiff, Hell Hound, Shadow Mastiff, Blink Dog, Werewolf hybrid form
That effectively merges in form of the icy hunter but it's probably okay.

[frogglesmash]

That's right -- well, unless you're converting lower-level spells, like the new [Polymorph] subschool spells, which might be "only one form" as a specific power limitation.

The arguable top 9th level spell (shapechange) might not make a good Invocation.

For alter self, maybe something like... when you choose this Invocation, pick 3 specific forms. You can assume any of these forms, or your normal form, as a Standard action.

For aspect of the wolf, maybe do something like the spider form invocation, and get better wolf forms as you level up:
- Level 1+: Wolf
- Level 6+: Dire Wolf
- Level 11+ (pick one): Winter Wolf, Yeth Hound, Nethersight Mastiff, Hell Hound, Shadow Mastiff, Blink Dog, Werewolf hybrid form
That effectively merges in form of the icy hunter but it's probably okay.

What I'll probably do is limit them to a few common forms+anything they encounter, and then have them research anything more exotic.

[Nifft]

What I'll probably do is limit them to a few common forms+anything they encounter, and then have them research anything more exotic.

That's a good way to encourage & reward adventuring.