well i like the last theorie
that would mean titans used to be human
which would explain how eren can turn into one after that injection
it is serten now that his healing abilitiy even funktions without a titan nearby
(the missing tooth that grew fast)

besides it seams that the 50+ titan seams to have sanity as well like eren in his titan form
and there is that woman titan who seems to be like eren a human-titan
but how did she end up like that
the father of eren again?

You are right. This seems plausible.

I am also guessing that the titans were developed by human scientists in the past. They are some sort of advanced humanoid weapon and were used in a war, possibly a world war, before. But something went wrong somewhere and they could not control the giants anymore. Maybe the users went insane? Anyway, Eren's father got knowledge of this and injected him with a substance that made him able to use such a weapon again. After doing this, Eren's father got captured by a power hungry faction that wants to overthrow the current government and enforce their own policy. Therefore they forced him to make some other human controlled giant. The result was the woman titan. No clue about the colossal one though. Their plan is to damage the wall. Which will serve the purpose of scaring the general populace and inciting a revolution from which the their faction will emerge victorious. Perhaps they are even aiming to be false heroes by letting the woman titan attack them. So, that they can defeat her easily because she is one of them.

Eh... the giants appeared a long time ago in the story's timeline... way before there were scientists capable of inventing a drug.

"I'll shut your mouth~~~~~ with mine~~~"

二息歩行

Quote from FormX

Eh... the giants appeared a long time ago in the story's timeline... way before there were scientists capable of inventing a drug.

Yes, but maybe the current story is playing sometime in future. You know, like this:

humanity is prospering > technology is advanced >some war > scientists develop a drug which turns humans into weapon-like giants > giants get out of control and attack them > the humans build the wall > a thousand years pass, humans devolve in the process > Eren's father discovers the drug again > he uses it on Eren...

I think it is similar to Cronin's novel "The Passage".