To me, it looks like PVP is going to be 70% wildcard for humans, and 60% wildcard for everyone else. Of course, that's true for your opponents, as well. If someone has a weapon, you can drop the number of skills you need to worry about to that subset - about 30-40 skills, and the spells that work well with them - reducing the uncertainty by another 20%. Wizards, I agree, are going to be much scarier over all, due to their high variability.
The only thing I know of that gives any sort of reflection is the Erwydra's alt form, which has spell reflect for the duration - and that is visually easy to discern.
(If PVP is your thing, an Erwydra Wizard might be your best initial bet, honestly, since they have a better number of options as a general tool kit, that alt mode reflection, and the capacity to GTFO if a match-up goes badly. Just hope you don't get stealth ganked by a rogue - cause Erwy mages are squishy squishy boys)