Evasion is probably one of the most useful individual autogrants, and often the main reason why a build takes /2 monk or /2 rogue. It pretty much changes the way a character approaches magic damage, and as such can pigeonhole many builds.
I think it'd be useful if there was a source of very temporary Evasion, enough that non-Evasion characters could make limited use of it, but without making Evasion classes largely obsolete.
My thought was a Potion of Evasion that simply grants you the feat, available from a vendor (maybe with a favor unlock, ie The Twelve?) at a price that's more than negligible, that lasts for 20 seconds. Maybe with an extended cooldown on top. That would be enough that characters could use it to help ameliorate one nasty trap, or provide a little extra defense in a fight with lots of AOE spells spamming around, but not so long or so cheap that you could use it continuously for perma-Evasion, or use it in longer fights except as burst defense. And actual Evasion classes would still be the only way to get Improved Evasion.