One aspect of game that feels underwhelming to me are victory conditions.
Closest to "hardcore" victory for any 4X game, with only difference that killing sovereigns destroys their nations.
2) Master Quest
Too easy to start. Usually, adventure tech tree is beneficial whatever you do in the game (conquering, expanding, building up), due to amount of gold that can be unlocked through goody huts, so even if you don't beeline for it, at one point you'll be tech or two away from activating it. And techs aren't really that much expensive to get.
At that moment, you get the quest, do some battle and win the game. Only potentially difficult battle is at the end with the hermit, due to him having 100 defense, but if you have some true damage attack (dragon, vigilant demons), it's not really difficult.
3) Diplomatic Victory
Get alliance with everyone. This was cheap victory in GalCiv2 too. Only challenge here is getting contact with all major factions (can be tricky on large maps), and having bigger army then anyone else. After some time, everyone will love you, and there is victory.
4) Spell of Mastery
This one is a bit more balanced end goal. Unlocking high level spells takes a lot of time, so without having good supply of arcane temples and all types of shards it's not practical to go for it.
Still, if you do meet all conditions, there is not much to do. Just end turns, build up empire and that's it.
On plus side, if you do not meet all conditions (not having all shards, low on arcane temples), this could be a good reason to get into late game war, which can be fun.
To sum up, if I would rank victory conditions from cheapest to most rewarding this way:
-spell of mastery
Ways to improve them:
Just because you have big military doesn't mean that other factions should like you. There needs to be serious diplomatic investment in getting other factions to like you, instead just building more units.
The main problem is that it can get unlocked too quickly. Exempt sufficient tech, there really needs to be some other condition added that can't be achieved too quickly. Maybe counter that will track amount of notable locations visited, and when it passes some point, it makes it possible to find location of ancient ruins where forge is.
Also, sub-quest locations really need to spawn all around the world, not just in territory near player (which can be bypassed with teleport). Quest really needs to feel like big voyage. Think of Marco Polo voyage and his life in China, and how many years it took him.
Spell of Mastery:
This one is fun, if you don't meet all conditions, so you need localized war to get missing shards or snatch a few arcane temples. But after that, it's just a big wait till the end. There needs to be something else to do at that time.
Also, there is small balance issue with discovering spells. Unlocking any spells of Nth level costs same as unlocking new N+1 level. It shouldn't be like that, since it makes 8-10 spells highly undesirable.
Only one thing. Killing sovereigns should not end their empires.