First of all,
Choose an interesting topic (period of history) which fits the style of gameplay mechanics already implemented in previous game.
Going to next military advancement will lead us to WW 1 or 2. This will add new gameplay mechanic such as planes (I could not imagine a game without them) and might increase the size of the level. Even though It might be possible to make it good, my heart is always in ancient times such as Roman Empire or Medieval. This is why I started playing AoE at all.
My suggestion of period:
Make a game where player has to choose continents, then regions and after 2 ages, he could choose a more specific country or empire which emerged in that time and region. This would let players experience periods of all previous AoE games in just one game. Times from 1000 B.C. till ~1860. This would UNITE ALL AGE OF EMPRIES FANS TO ONE GAME. But of course, it would bring some challenges, such as making a smooth transition and balance all the countries at different periods. I suggest making some strong at one age, while others at next ones (similarly as in AoE 3). Later on, leave options for players to lock and choose which age they want to start from. This would let aggressive players to play for their favourite civilizations (if developers still want to keep the game fast paced).
A more in-depth explanation:
First, pick up the continent:
Europe, Asia, America (natives) and perhaps Africa.
After that, pick empires which existed in a certain period of time, for example, in the starting period, let players choose between Roman Empire, Greeks (just for Sparta ;D), Ottoman Empire, Egpyt, Mongols and more.
Let them play for 2 ages, after that, let them choose specific empires or countries which existed in later times but in the same territory or close to it.
So if someone picked Baltics, they could go for Lithuania (which was the largest country in a certain period of time in Europe). If they picked Roman Empire, they could go for Germany, France and others.
Once they go to age V and VI, they could choose more modern civilizations, such as featured in AoE III.
The transition from empire to a country could be done before the match. I suggest doing it real-time to make the game more dynamic and less predictable. Usage of cards (if still kept) could be simplified slightly as it would already be quite complicated. Which deck to choose might be decided real time as well.
Make more visual options and shaders. Stronger anti-aliasing, more options to zoom out, more intuitive controls.
But most importantly, gameplay options:
We need more advanced A.I. with more options for players to choose from. Make difficulty scale not in 4 (easy, normal, hard, expert) steps but 10. For me personally, expert bot is too difficult while hard is too easy. There is no middle ground for my skill.
In the very least, give option for players to choose enemy’s playstyle, such as aggressive (rushing very early), medium (hybrid, growing economy but still keeping pressure) and passive (focusing on defences and doing attacks later (turtle or boom)).
PLEASE FIX A.I. ESPECIALLY IN NAVY BATTLES. It’s utterly broken in AoE 3. It’s not even challenging.
Improve map editor (if not, at least charge me 2-5 dollars to unlock it. I would pay that JUST TO GET A HIGH QUALITY MAP CREATOR), so making maps would be more efficient and easier. Giving community at least majority of tools used by developers. Give option to add custom textures, models. Making exporting the levels and sharing it with others an easy task.
Let players host different servers with different game modes and rules.
Add MATCHMAKING with teams (+ voice chat) or solo and ranking system (similarly seen in CS:GO or other competitive games) which could be determined in the start by 3 games against a bot. Bot difficulty would be chosen by the player himself. After 3 games against bots, he would need to play against 5 other players to confirm his ranking and elo points. This would make the community more engaged and multiplayer gaming more fair.
Obviously, try to make civilizations balanced. Listen to player’s feedback, perhaps give them to fill some surveys in-game or just integrate to the engine itself statistic gathering algorithms to let you know what is being abused. There is no fun if one civilization is OP.
Additional categorization of Units: similarly to AoE 2. Make them march in a square, line, spread them out, group them together and etc. Make more possibilities for stances and attack targets.
Add macro keys to build specific troops automatically from barracks, ships and etc. and another key bind to place the location where to send those troops.
Give option to separate speeds for troops, so if one type of units are faster, let them go to the same point but don’t slow others down.
Rewinding of recorded games more easier to watch. So you could choose a specific time / age / moment through timeline and go see it. Skip or speed up more (more than twice) specified parts.
Ability to coop – two players either in lan or multiplayer play as one. It would be much easier to control military and economics in the same time, especially once player reached over 100 pop.
Weather systems such as rain, snowing, sea storms, black fog (everything is dark unless it’s in unit’s vision), sandstroms and etc. Make an option for players to select weather while playing against bots or in multiplayer matches.
Ability to Customize the U.I. and HUD with at least different colours, sizes.
And finally, perhaps increase max population size, especially in the last age. 250 would be a decent number. Make improvements somehow visible on units. Add a lot of playable maps with different setting and mix of land and sea, some extremely balanced (for very competitive players) while others more dynamic.
PLEASE FOCUS ON QUALITY. Don’t rush. If needed, add less DLC but do higher start prize. We will buy the game if it’s good. Please do not follow steps of EA, Bethesda or other money milking studios. We are not interested in paid skins, we want gameplay to be top notch. It would be EXTREMELY nice, if you added a demo version of the game with multiplayer included.
!!!Please consider this survey!!! to help with what players need and expect. Good luck developers, I hope you will make a new bomb in RTS games and the best title from AoE.