so after rereading chapters 60-71 i was thinking of something; before these chapter's eren couldn't harden at all, but after swallowing a bottle of some sort titled "armor" he was able to harden, this brought me to a conclusion that you could add abilities to a titan by adding a serum to him. similar to how one becomes a titan (injects a titans spinal fluid) if a already shifter was to swallow/inject a different titans ability he would have been able to gain its powers. if this theory is true there could also be a serum that lets your titan have armor, be giant, cover himself in crystals or have a ton of hair on you. but if we take this a step further, one might even produce a shifter serum that lets you shift...
Interesting, so far as the plot goes no one have acquired control over its titan form without eating a shifter.
If you think about it, Eren took a shifting serum, he just was turned into a mindless.
But what you say make sense to me, if there's the chance to pass other abilities, as hardening via serum, why not the control of the transformation?