The 5 general "paths to victory" are:
1. Military- Players should focus on military techs, designed units and large trained armies and will be the most effective in fighting other players. Gold goes to unit wages and upgrades.
2. Magic- Players should focus on the magic tree and capturing mana shards to increase the rate they gain mana and to make their spells more powerful. Players are the most effective at short term powerful effects. Mana should be spent on expensive power spells like Summons, Blizzard, Touch of Entropy and Grip of Winter and not on spells that boost other focuses like Inspiration and Crusade.
3. Civilization- Players should focus on Civilization techs and investing in their cities. This is the path for turtle players and is where I am focusing at the moment. Players should be the most effective at getting at the late game buildings, building population, building gold, and fueling other aspects of the game (many players will focus on Civlization to some degree, most of the cross tree tech requirements are out of the Civlization branch).
4. Adventure- Players should focus on the Adventure techs, recruiting champions and completing quests. Mana should go to unit enchantments and imbuing champions. This can be the least predictable of the paths, with random magic items providing rewards from minor boosts to powerful effects. Quetss grant experience to champions that complete them giving players an avenue to level their champions before going to war with other players and champions are the best able to deal with the powerful monsters of the world that have traits like Overkill. Gold goes to recruiting champions and buying items from shops.
5. Diplomacy- Players should focus on the Diplomacy techs, claim areas and build improvements that give diplomatic capital. Players should be looking to build camps in captured monster lairs and convince AI players to help them. Players who invest in it can get access to some unusual creatures for their armies, including dragons and are better able to maintain peace with other players while they focus on other pursuits (such as completing quests or exploring for uncaptured mana shards).
The above are the paths at a concept level. It's our job to take the above and enforce it with mechanics that give appropriate costs and rewards for all of the above. It isn't intended to be a direct implementation of the above, and we don't expect players to only follow one of the above.
Players may pursue Magic and Adventure to recruit and imbue as many champions as possible, allowing them to tromp across the map and complete quests with summoned henchmen and largely ignore the military side of the tech tree. Or players may rush to get the appropriate military and magic techs that allow them to start crafting magical armor for their trained units, fielding armies of very expensive, and very powerful soldiers. Or players may decide to turtle inside their super cities, managing a few groups exploring the world while they concentrate on building the cities that will provide all the people, wealth and resources they need when the time comes.
The above determines where a game mechanic belongs. Cities don't generate mana, they come from mana shards. The reason is to make civilization focused players and magic focused players different goals. If mana came from cities then the civilization focused player would be the best magic player. Champions and armies have to have distinctly different advantages and disadvantages because we don't want players to simply be able to trade one for the other without a significant difference.
Spells shouldn't simply be another way to get at building effects. If we do have a spell that does something a building does it's for a specific reason (it goes against the general rule, which we do all the time if it makes sense in the details). For example, Inspiration provides bonus research to a city much like a Study. But Inspiration costs mana. In this case it isn't that we are giving Magic a civilization type bonus, its actually a spell for non-magic focused players. If you are focusing on Magic you probably shouldn't cast Inspiration, save that mana for the bigger spells you will want to cast. The spell is for players that won't invest in mana and want to trade that game aspect for help in other areas.