The question of how to fill the 'wizard' niche is fairly important, and the answer isn't necessarily obvious:
The simplest option conversion-wise is to use David L. Pulver's Eidetic Memory article 'Magic as Psi,' in Pyramid Vol 3 #29, pp16-18, which was mentioned in the OP. It's a good article and an interesting idea, but not that much of a change from the source material.
Another option, simple from the psionics side, is to declare that 'wizards' are those who specialize in using their psi abilities (especially the more versetile ones), or in having lots of psionic Alternate Abilities. See also the 'Old-Time Psi' text box on page 5 of GURPS Psionic Campaigns. You aren't limited to the standard powers from the Psionic Powers book, as long as the GM is willing to accept the power talent you come up with. A hypothetical 'Necrokinesis' power would be quite useful for a GURPS Banestorm: Abydos campaign - perhaps adapting the Death power from GURPS Powers p125 (and adding a few more abilities, like Telekinesis with the 'Animation' limitation from Powers p83 and a 'Corpses Only, -30%' limitation, or PK-Binding that requires the presence of dead bodies to hold the subject), and psi-based Elemental powers would fit the mentality of the setting, even if they would also mostly be specialized versions of PK, with a few related abilities added (e. g. Exoteleport fuel into fires). I might make some worked examples in later posts. This has the downside of being pretty expensive in comparison to the standard magic system, even with the Alternate Abilities rules, though. I've considered a couple of variants of this, which could work in combination:
- Wizards are Mystics with lots of alternate abilities, and the canon mage guilds are secular or semi-secular mystical organizations (and some of them are not secular at all), many of them with 'Pact: Ritualism, -5%.' This does mean working out more details than you otherwise might on each guild that's going to be important, and some that aren't, as well as helping with player-created guilds, but you might enjoy that. Creating Lazarine Mystics/wizards (see GURPS Banestorm: Abydos) could be interesting, for example.
- Take Sorcerous Empowerment, and replace the 'Magic, -10%' limitation (which it at least has in the Pyramid Vol 3 #63 article, I don't currently have GURPS Thaumatology: Sorcery) with 'Meta-Psi, -10%,' or perhaps with a specific 'regular' psi power (so, you have Psionic Sorcery (Telepathy), Psionic Sorcery (Biokinesis), and so forth). You could also adapt some of the Sorcery colleges to psi powers, but then you're getting pretty close to 'Yrth as a Sorcery setting,' which could be interesting, but is a different thread. Basing it off of another form of Modular Abilities can also work fairly well.
In any case, this is fine if you don't mind 'wizard' characters being some combination of 'low in power levels,' 'rather specialized,' and/or 'rather expensive' (and indeed, a 200- to 300-point Yrth game could work quite well), but there are other possibilities.
Another option is Spirit Communication. Spirits are canon for Yrth, so it's just a matter of calling them psi-based spirits (astral entities), rather than magical ones. From there, one relatively simple 'magic system' is to just use Spirit Communication and Influence rolls - along with Occultism or Spirit Lore (or another appropriate skill or skills) to work out what spirits to contact for what purpose, and perhaps 'Spirit Empathy (Astral Projection, -10%) [9]' to hopefully make them more agreeable - to get the spirits to do things for you. Spirit hirelings, as well as Allies and Patrons, are easily plausible (spirit Dependents are possible, but odd), and a wizard would generally be one who has a lot of them. Ghostdancer did a helpful chart for ally/familiar/dependent costs
in his blog (scroll down a little), which I highly recommend. This does rather add to the GM's burden, though, as this system is very, very freeform. A couple of variants of this (one of them rather less freeform) are below:
One way to use spirit-magic is to adapt the existing Path/Book Magic system from GURPS Thaumatology (pp121-165). The Paths and Books could be described as spirit-focused social skills combined with specialized Spirit Lore/Occultism skills, while the rituals are contracts that usually work with those spirits. Magery 0 is replaced by whatever level of Spirit Communication you have, and the Magery talent is replaced by the appropriate Psi Talent (by RAW, Spirit Communication is an ESP ability, but I prefer to put it under Astral Projection or Telepathy). Path/Book Adept probably either is not available, or has an appropriate power limitation. The base skill could be Spirit Communication, with Ritual Magic (or an equivalent, like Symbol Drawing or Meditation) as a Complementary Skill, or the Spirit Communication Ability could be operated by the Ritual Magic skill (or, again, another appropriate skill).
Going another direction, you could use the method from the
Spirit Magic as a psi ability thread: To summarize, you have so many spirit allies that they are mechanically represented by a Modular Abilities advantage (generally with a limitation like 'Only for Allies with Summonable and Minion, -50%,' 'Only for Allies with Summonable, -40%,' or a specific category of spirits, generally adding a further -10% (or worse if it's especially narrow or otherwise disadvantageous), and a few others, usually including a skill limitation). This may be because you contracted or made friends with them before the game starts, or during the game if the GM lets you gain the advantage in-play, or because a Patron or other such being helped you, or perhaps because you happen to be very good at generating thoughtforms at a useful power level (
tulpa; see also GURPS Horror for 4e, p81). If the advantage is granted by a Patron, a Pact limitation may apply. Categories of spirits might include Demons (there are always literally-damned fools out there willing to sell their souls for power), Faeries (almost as dangerous as demons, though for mostly-different reasons), Ghosts (depending on the GM, this may be a fairly safe option, but not a greatly powerful one), Nature or Hearth spirits, or something more specific, like Animal totems, Plant guardians, or Elementals (e. g.
Elemental Phantasms, from the
Echo Type Ghost Template thread - itself pretty useful for this setting).
A different option is that wizards are those who have and use lots of psionic items and/or psi boosters. This may mean that they are fairly rich, that they have a patron who provides the equipment (e. g. wizards in military service), they they are a very skilled and daring thief, or that they are highly-skilled alchemists, herb-lorists, or attuner, or some combination thereof. I don't think I've seen something like this used in a game, but Robert Asprin used it in
Myth Directions for Massha when she was introduced, before she became Skeeve's apprentice (which I think was in a later book); Asprin calls her a 'mechanic,' and that may be an appropriate term if this type of wizard is present along with one or more other types. Visually, this sort of wizard is likely to be fairly similar to a Path/Book Magician, in that when adventuring, both would be carrying lots of fiddlybits: jewelry, small bottles, weird-looking geegaws, and so one, many of which might still be single-use.
There are probably options I haven't thought of, but this seems like enough for now. As noted above, it's reasonably plausible that more than one type of wizard might coexist in the setting. It just depends on what sort of game you want to play.