First of all: Incredible work! Unbelievable that You did this all alone. And thanks for taking into consideration players' suggestions.
I tried V 1.2.1 at easy level in campaign mode and completed all the missions that are available in the beta versions. Today I gave it another try and completed the (demo) campaign on hard level.
I didn't find any bugs. Some thoughts on how to make the game even better:
All the battle-maps are designed more or less in the same way: a street in the middle and some houses on both sides. This can get boring after a while as it allows players to use the same tactics on about all the maps.
I think you could add greater variety to the map design without even creating new artwork by just balancing houses, streets and landscape differently on each map. With the objects included already in the game you could create a map without the notorious street in the middle and set it up for example with only houses and narrow paths between them. This would create a sort of urban warfare battleground allowing players to take advantage of light wapons. Other maps could be forests or fields, consisting of only bushes and trees, forcing players to use different tactics (and weapons) on each landscape.
The mission objectives could me more variable. What about taking/defending a certain object (building or bridge)? What about hostage rescue or protecting civilians / refugees? Or giving cover to a person that has to walk to a certain point on the map? The person could be an unarmed member of your team, allowing you to control his movements in these missions...
Grenades have already been demanded. I would like to add a sniper rifle, smoke and flashbang nades to the wishlist, providing new tactics to the players. And what about a rocket launcher? (remembering the good old x-com days...)
A way of healing your soldiers without having to drive to the hospital all the time would be nice. Continuous self-healing or pop-ups offering medical aid for cash would be an option. What about medi-packs in the game?
I am collecting loads of useless weapons and have to drive a long way to sell them. You could also add pop-ups with merchants offering to buy those weapons, maybe for a worse price but without having to drive to another city.
You could add more interaction with allies. They could request assistance for attacking certain places. And players could request assistance for their intended attacks.
I would also apreciate (as other players have mentioned here), to be able to fire although the enemy is out of range. There can be reasons to shoot although the hit probability is very low. For example when a group of enemies is approaching you may hit any of them when shooting at random. This would also enable you to draw the enemies' attention to a certain player (in safe distance) and leading them to an ambush you prepared for them.
Reverse Angle Camera: works great, but what about more angles? or even free camera rotation? When you are in a building sometimes none of the available angles will work for you.
Diffculty: On easy level most of the missions are a piece of cake, that's ok. But 2 or 3 cities in the middle of nowhere are defended by about 2 or 3 times as many defenders equipped with powerful weapons which makes it hard (i think almost impossible for novice players) to win these battles.
The hard level could be more tricky. Once you learn the AI's tactics it is easy to outskill them. You just have to build your fortress in a building or behind a wall with all your soldiers assembled at one place - and let the enemy attack. They will do the favour. After some hiding they will start to run uncovered into your ambush and you can take them out 1 by 1 (which ist fun anyway :). Just stand up, shoot and crouch again. Your soldiers are almost invulnerable behind objects. Those enemies who manage to make it into your fortress will be killed by one soldier who has saved some APs for exactly this purpose. This tactic allows you to collect the assault rifles your dead enemies leave on the floor although your soldiers were only armed with pistols. Suggestion: AI (enemies an allies) should try to take better cover and not head directly to their opponents but look for a sneaky way, where you can't spot them or where they are covered.
7 enemies attacking 3 mercs armed only with light weapons. I tried to hide in the building some times but they came from everywhere and kicked my ass. So I restarted the battle some times and tried this place. For whatever reason the AI in this case doesn't try to attack from various sides. All enemies stay basically at the same spot and wait for me to take take them out. 
My soldiers move one field towards the street, fire and hide again. The opponent's bullets all end up in that fence/wall where I am hiding. One remaining enemy approaching from the top comes too late.