Great war game!
This is definitely the start of something!
Instead of saying how great it is, i'll give you stuff you can use.
- Allow more user control: Choosing where one invades would be fun, and each country could have various difficulty levels.
- Don't make it so that on level 21 the player looses units without actually loosing them!
- Make other countries playable - the stronger/larger the country is the easier it is with fuding, etc. (US = easy while Poland = hard) or something along those lines.
- Make single mode more customizable (of course after player has unlocked specific units.
- Change up air battles! They require no strategy at all - just build build build units. Plus - they are not easy to win at the same time. It's all chance. Make it first person or mission based perhaps. For example, you are a pilot and have to bomb a base.
I hope you find some ideas useful! If anything please consider the multiple countries one can play as and choosing where one invades.