The tech tree right now has several problems. Firstly anyone smart would always go down the civics tree first because that's how you get more research. Secondly it takes forever to get 5th and 6th level techs. May other concern is that when it comes to looks and items you can't really customize your heroes or units until late game. You always end up using the same armor and all your heroes look the same. There is a cool weapon versus armor bonus dmg system in FE, but it never gets used because it takes so long to unlock the different armors. This is because there is only one type of each armor with no progression. More early game armors are needed to increase strategy ans unit design.
Add an armor called Splint Mail to the military tech Leatherworking. There is currently a huge gap between Leatherworking and Armor. Splint Mail would be a new heavier version of leather that would look bulkier and have metal bands crossing it. It would be heavier and reduce dexterity in return for more armor. It would also cost some metal and add blunt defense. Leather would also gain piercing or cutting defense. Thus you would have a choice in early game armors, light units would wear leather and and heavy units would wear splint. So all your Assassins and Warriors wouldn't be wearing the the same armor.
Add a lv3 magical tech called Arcane Tools. It would allow you to buy Hooded Apprentice Robes and Staffs for your champions. The robes would be hooded and boost spell mastery by 4 and armor by 2. The Staff would be a normal staff that also boosts spell mastery by 3. This would allow you to equip your mage heroes early game with dedicated mage items, instead of them wearing leather armor.
In my old WoM Tactical Balance mod I used hooded cloaks to great effect in regards to making light units look unique. This wouldn't require any new art but would really make dexterity champions look unique and cool. Think about adding some of these type of items that boost dexterity and dodge. You could also add them to the magic tree somewhere so you could purchase them.
The thing that slows down mid game research is the two tier gap in the civics research tree. You can't have increasing tech costs but not increasing ability to research, or you will stall hard. Either add another research building and tech to the civics tree or better yet to the magic tree. If that still doesn't work reduce the research cost of techs past the 4th level by a bit.
The magic tree could really use love, it's mostly useless. Tech is and always will be, disregarding champion cheese, the most important resource. If you make the civics tree the only way to get it then you are forcing everyone to focus on civics. So add a 4-5 tier research building to the magic tree, make it require crystals or consume mana. Or better yet since magic and champions are tied together maybe make a building that takes a garrisoned champion's intelligence and turns it into research. It would probably be a good idea to add more then one magic research building. Reward players for choosing to invest in the magic tree by letting them be better at researching even more magic stuff, if you don't it will always be secondary to civics and everyone will use the same tech tree path order. The same goes to a lesser extent for the military tree. Maybe add a tech that generates research by conquering cities and killing enemies. Or an improvement that gives a small research boost in garrisoned cities, or only if you are at war.
Move the first few creature lair techs down a level or 2, and possibly move them to the magic tree.
These changes would greatly increase the ability of players to choose their own tech path and equipment for heroes. Champions would look far more unique in regards to how they are specialized. It would also add a lot of tactical depth to the game because you would actually have to use certain weapons against certain armors early and mid game.
