"We know about a hundred meaningfully unique magic systems, but many of them don't transfer well off-world because they rely on local physics or resources. Let me see. For themes there's Druidism, Skaldwright, Wizardry, Psionics. Sorcery and species magic. Clerical magic like that practiced by your mages, though from the fragmentary reports I've had of Finnish mages they sound like they could be Sorcerors. Inspiration and Awakening - both nasty to handle around non-mages. Artificing and Summoning - both technically forms of Wizardry. Persona and Incarnum. Naturally occuring magical items. I could go on.
"The different kinds of magic require different skills, but the ones that transfer well without needing strange prerequisites are Druidism, Skaldwright, Wizardry, and Persona. Druidism is based in attunement to the natural world - not too dissimilar from the kind that you learn when you're doing ranging. If you have a ranger who also gardens, they'd be a good Druid candidate.
"Skaldwright is the art of master poets and musicians. It comes from charisma and a love for verse. It's basically songs and poetry that, when you perform them well enough, do things. If you have performers and garrulous sorts, they'd be good candidates.
"Wizardry is book learning by rote and being able to visualize complicated objects in your head. It's complex, fiddly, and takes years to get anywhere. I wouldn't recommend it to you. Maybe there's some bright spark here who'll love it, but it's a magic for soft people who have all the time in the world to study and learn. Druidism you can pick up in the field as you go along, and skalds have been skalds since forever.
"Lastly there's Persona magic. It comes from knowing yourself deeply and having a good relationship with your own soul. I'd recommend it to veterans like yourself. You learn a lot about yourself being in dangerous situations."