Right now each of the factions, races and sides (Kingdom or Empire) are far too similar - they're almost copies of one another and the differences have far too small of an impact on gameplay.

Designing certain factions around specific gameplay styles (such as focusing on fast hitting small armies, slow powerful massive armies, offensive magic, defense magic, enchanted armies, etc) as well as the different races and the two actual "sides" (kingdom / empire) would be great and would allow players to mix and match the three components to produce interesting results. The biggest benefit would be a greatly increased replay value.

Rough example:

• All empire units get +1 strength, move more slowly on unblighted tiles, and get unique offensive techs.
• All kingdom cities generate populations more quickly than empire cities, kingdom cities require 10% upkeep, all kingdoms are able to build armor more cheaply.
• Faction A gets military research cheaper, can produce units faster, but all units have 15% less health and cannot use magical items.
• Faction B units get increased attack when using mounted animals, air shards generate 20% more mana, other shards generate 15% less mana, unique air magic spells.
• Faction C cannot research much in terms of armor, but gets +damage when using ranged weapons and has unique ranged weapon tech, receives -10% mana from all shards and magic research takes longer.
• Race A units heal 25% faster, magical effects on units are reduced by 10%, suffer -1 movement during summer and spring (or equivalent seasons). Unique death magic.
• Race B armies require increased upkeep. All blunt damage reduced by 20%. Unique earth magic and increased potency of earth magic spells.
• Race C buildings are 20% cheaper, 15% faster to build but 25% weaker.

Using the above examples, a game would become instantly more interesting for the player. Encountering enemies consisting of new combinations would make the game more strategic, causing the player to play to his/her strengths in order to eliminate enemies.

Also if you give bonuses based on season, it might be worthwhile to make a season 2 turns (or 3) long.