I think that there should be more interrelation between the tech trees. For example in the magic tree there are the arcane weapons and armor techs. Replace the two techs with Arcane Arms and give access to "Legendary items" only for those that you've opened up through Warfare techs. Like Legendary Leather Cuirass and Legendary Short Sword if you've only got the basic Warfare item techs but as you get the "blunt weapons" tech you gain access to Legendary War Hammers.
Or you can ask various arcane and magical creatures to join your nation in the wild if you have the right arcane and diplomacy techs.
An Antiquities Lab building opening up when you discover the civilization tech that provides University (higher learning?) and an appropriate Adventure tech dealing with ancient legends.
Bountiful Harvest becomes available when you get farming guilds and an appropriate Magic or Adventure tech as you've discovered that combining advanced farming technology and ancient or magical compost/herbs that the plants grow faster and larger so you get tomatoes the size of cantaloupes or whatever.
The point being is that cool or odd combinations of different techs or even Arcane lore like the sovereign learning level 8 spell knowledge (representing a great understanding of the magic in the world and how to manipulate it) would add variety and many different equally valid paths to victory or even just a little flavor or style to different players.
I also like the idea of requirements from other tech trees but I like the idea of limited effect better like the above Legendary Items example in which you can research it but it works best in combination with other knowledge.