I'm looking for any chiddushim the Maharatz Chiyus wrote on the Agadeta. He wrote a "Mavo HaAgados" in which he lays down principles for learning Agadeta and I'd like to see how he puts them into practice.
On many volumes of the Talmud there are printed glosses (גליון) from Rabbi Chajas.
His comments range the Talmudic spectrum including Agadata. (Though, it is slightly unclear what you intended to include with that word.) However, his general style is to reference all of Judaic literature in order to explain a passage and form ideas.
It is worth mentioning that he himself prepared his glosses. As he writes in Shu”t Mahartz 3.1 (in the כל ספרי מהרי׳ץ חיות pg. 619):
כי אחר איזהו שנים בשעת הסדרת הגהותי על הש״ס
In fact, his glosses precede the מבוא התלמוד. In his introduction to his glosses at the beginning of Brachot he writes:
ודעו גם כן כי כבר התחלתי לסדר מאמר מבוא לתלמוד להקדימו להגהותי
There is a two volume set of all the writings of Maharitz Chayos called כל ספרי מהרי׳ץ חיות.
In volume one is his introduction to the Talmud. Chapter 17 (pg 315) and on discusses Agadata at length with various examples from the Talmud.
See there also page 243, דרכי ההוראה חלק שני, where he discusses which Agadata can be used to rule halacha from.
There he references at least one more work of his on this aspect of Agada. It seems like it might be called אגרת בקורת, but I don't think I've seen that work.
If these selections aren't what you were looking for, good luck on your hunt!