Shorter title for the post, please. Its too long and most people will not see all of it until they open the post (assuming they do so at all). You're supposed to sell the post, not try to rush explain the whole thing.
What if SD develop the game differently, so that more land does not automatically means more power?
Currently, they 2 game mechanics that are supposed to slow things down.
-First, placing cities without fertile land costs essence. Spending essence in this way will limit maximum mana capacity of your soveriegn, thereby limiting the mana you can use for your spells. You may even run out of essence they by preventing a player from placing towns just because they felt like it.
-Second, more towns increases maintaince costs. Too many towns and you'll be feeling the financial pinch.
Consider using a different name for what you call "Achievements". There are a lot of games out there that uses that term for player achievements. Basically, its a system of braging rights, or stuff for a player to do. Using the term "achievements" keeps getting me thinking about that, and not a bonus system for units and stuff.
Try "Unit badges", "Traits", or just plain old "Bonuses".
Achievement based bonus for buildings:
How about all your “Academy of Fire” will produce 200% additional research points when you empire has 2 active volcano? (so you want to cast a volcano spell on your own land…) Or maybe your mana node will produce 300% more mana if you managed to stay in peace for 50 turns?
Sounds more like a circumstances bonus than anything else.
Say this sentence out loud "Do I lose my fire research achievement when the volcanos burn out?".
Try this sentence instead "Do I lose my fire research bonus when the volcanos burn out?".