I want my Doppelganger villain in my D&D 5e game to be a worthy match for my level 7 party, but since it's CR 3, if they get it into combat, though it will be fun, it won't last long.
My goal is to improve my monster so that the monsteriness and the classiness of it are well-integrated: so it doesn't feel to my players as if their antagonist is a generic rogue NPC, but a weird extra dimensional being, but also that they can't mow it down immediately. (Of course, roleplaying is a big part of this, but I'm consulting about rules and stats specifically). I'd be all for homebrew rules.
Is there a good way to add a few character levels to my Doppelganger-- say four Rogue levels-- to fix this problem? I think that should boost its power appropriately, but will it actually do so?