As I was walking home from my university I started thinking about conlangs, as I usually do. I find the way home very inspiring. I was remembering my first attempts at cracking Japanese, when I only had a japanese video-game and a kana chart. I was thinking about the copulative, which really got my attention back then. The copula can change depending on register, from です to だ, without mentioning the expletives and emphasizers.
I rekindled one idea I used to have. In japanese you can say スタンドだ (sutando da), which means "it is a Stand" or "A Stand!" (this is due to the game in question being Jojo's Bizarre Adventure). But you can also say 美味しいです (oishii desu) "it is tasty" or "this is good!" depending on context. So you have in the first case a noun and in the second an adjective, so I started to think... how about a conlang with a copulative particle which changed depending wether it is a noun or adjective or whatever.
So for instance, you could have something like... unil da, "(it) is love", but you could have mundo ki "(this) is old". So the copula could act as both a copulative and also a word classifier. Maybe you could even make the words flexible enough for them to mean unil ki "this is loveable" and mundo da "it's an old thing". Now that I think about it, it'd be as if you were to take the suffixes in Esperanto as separate particles... libr o "book" and libr a "book-like". But also mixed with a little Japanese :p
Of course I'm not saying I would use this very words or particles as they appear here, just testing the idea with some sounds to see how could it work, so this is not purely theoretical. Well, this was my idea for a conlang I had yesterday, just another idea...