I've just had an idea to further develop Glenn's personality (not that it was lacking, but more is better): Glenn is secretly haunted with the feeling of having no place to call home.
Indeed, he has been "lost" ever since 590 A.D. when his friend Cyrus died and he went into hiding in the wilderness as a mere frog-man. Then after the events of CT, due to King Zeal, he returned in an alternate 600 A.D. in which Cyrus was never a hero and the Mystic War still continues in 602 A.D., while people think he has delusional disorders due to his mumblings about the difference between his timeline and this alternate, Masamune-less timeline...
We can take this idea further if we assume that these stuff have some kind of taxing effect on Glenn's mind. Glenn has no home; he is foreign to everything in his era and thus has difficulties blending in society. Hadriel's idea of Glenn having a subconscious "hate" for the Mystics can even be resurrected here. There was a New Game+ ending in the very first plot outline about Glenn becoming the immortal emperor of the Vanguard and totally massacring the Mystics... perhaps this ending can be recycled into Glenn's Frozen Flame nightmare sequence.
In the main ending sequence, Glenn would not return to Guardia's Castle; he would prefer living a life of Wanderer in the wild. After seeing all the places and eras (and alternate eras) of CT and CT:CE, you can't really blame him for being tired of Guardia's Castle.
Of course, all this might look slightly less exciting if we're stuck with a "Frog" PC Glenn