I think that the Engine (Unity, Unreal..) and Dimensionality (2D, 3D, 2.5)

Very few games actually use from what i've seen the '3D' tag, so this might help..

It's already possible to  add an engine. Look at the game's metadata, there's a tab for that. As for what counts as 2.5D... the term has so many meanings as to be nearly useless. And if it's true that few people use the 3D tag, what makes you think they'd use a metadata field more?

I never before looked into the Metadata tab (primarily probably 'cause i didin't quite notice that, maybe would be better if it was in Bold Text)

I think that the 2D/3D option (the 2.5D or Text game would be as secondary options) should be in main "Edit Game" tab, many people don't realize that games shall be classified by tag as 3D (there are only about 8 times more games classified with the 3D tag than with the 3D platformer..).. For me it's perhaps just as important as the "Platform" & "Price" tag so far..

Also, i think all the games with the Unity/Unreal Engine tag should automatically move such tag into the Engine tab, it makes it confusing when there is both a tab & tag for it.. If such option was made, i also think it would be nice if all "2D" and "3D" tag games would move into their respective dimensionalities..