Honestly, I think everyone loves to overthink these things. How many times do pokemon change types upon evolution? A small percentage, thats how much. No need to get too symbolic with things. Every idea people have thrown around can be as easily done with the existing type as with any other, and any difference that would come about from dropping a type or replacing one with Normal (ugh) can just as easily be portrayed with Dark/Grass.
The only other real types that I think stands out as being reasonable is Poison/Grass or Dark/Poison. Personally I think the latter is better, but either would be OK. Now, obviously the movepool would be odd for that, but it is a reasonable change flavor-wise, and unlike dropping a type or trying to sub in Normal it actually represents something completely different than Dark/Grass. There is also the Skorupi/Drapion line that can be used as precedent, as it changes one type drastically upon evolution to one of the types we have here.
As a side note, can we please stop bringing up normal/something all the time. Normal adds nothing to flavor, so unless we want to be pure Normal, there is never a reason to have it at all. Something like pure Grass to Grass/Dark has the exact same implications as Grass/Normal to Grass Dark, and has much, much, more precedent. Being unique for its own sake is just silly and ridiculous.