For me, how I match up the pretender is different for different nations and themes (plus I still like occasionally trying something experimental in MP). It also varies according to game settings - what I want for a very difficult research game with Indy Strength 9 can be very different from normal research, Indies 5 or 6.
If a nation has very strong (or very
cheap sacred troops ( Vanheim, Man's Last of the Tuatha, Marignon ) I may try to pick a pretender that will give a cost-effective strong blessing.
Mostly though, I want a pretender that can act as either a SC or an artillery platform, while having magic strengths that are different from what the theme's national mages have. For instance, making sure I have earth or death on the pretender, if the theme doesn't have easy access. Broken-rainbows - 3 or 4 paths between 3 and 5, for instance.
Other people have very different desires / intentions for their pretenders, but generally you want to play to the strengths of a pretender-chassis, while compensating for a nation / theme's weakness.
Or you play to the theme's strengths, as Zen mentioned - a high earth magic Cyclops for Ulm is pretty potent in combat, and with research can chain-cast some potent battlefield magics like Bladewind with low fatigue costs. Add some fire to that Cyclops, it can do all the nifty magma spells. Or add some water, and it becomes a fair tank, with quickness and breath of winter cast. Of course - that also gets it all kinds of afflictions, so maybe you want nature as well, for regeneration and hopefully forging or summoning some things that can heal wounds.
Hundreds of different ways to pick and design the pretender, according to what you want to try with it. You just have to try and avoid some of the ... decidely sub-optimal pretender chassis's at the moment.
