This Is what he said during Q&A session (by reddit source)
Essentially Adam Isgreen (the creative director of AOE franchise) was asked about AOE4 and gave couple of vague high level answers.
He didn't confirm that they will reveal it at XO19, instead went along with the "wink wink nudge nudge" method that we got from Gamescom.
About development, he said that everyone (in-house supervising studio at MS, Relic, FE, and Tantalus in Australia) takes it super seriously to develop the games (3 DEs + AOE4). They want to preserve and stay true to that feeling which makes AOE what it is.
Coming from the background of developing C&C and Red Alert franchise, Isgreen mentioned about bringing mechanics and ideas from other RTS games to AOE, for example, right-click-and-drag map scrolling. He could not withstand the archaic method of moving the mouse to the edge of the screen. According to him, the new scrolling mechanics is in all the DEs, but not AOE4 yet which frustrates (jokingly) him a lot.
There's also mentioning of some innovations to spice up some mundane and boring part of the genre. One example he talked about is how the resource gathering is usually very boring, but in Red Alert 2 they made it more interesting by introducing different harvesting trucks to different factions (teleporting Allies, armored Soviet). Relic and he have had a lot of meetings to discuss what kind of ideas and innovations are worth the effort and budget to put in the game. They don't want to "bring change for changes' sake".
All 4 games had pre-alpha (maybe beta, I forgot the terminology he used) playtesting already. For AOE4, some die-hard fans of the old games were quite shocked and "intense" to changes made to AOE4 (sounds like AOE4 will be kind of revolutionary to the franchise). He used the analogy of "7 stages of grief" (I believe it should be 5 stages but whatever) saying that those fans became softer and more welcoming to the changes later on.
To summarise, he was extremely evasive on AOE4 saying that he didn't want to leak anything about the game (time setting, new mechanics etc.). But one thing is quite obvious TO ME: AOE4 will be a quite different game. Hopefully we can soon find out whether it can capture the magic of previous games or not at XO19.
