In practice, if I see an opponent attacking me early on in the game, I prefer to just quit and start another game rather than fight, because even if I do fight and crush the attacker, I will be set behind by the losses with nothing to compensate while the other un-attacked opponents zoom ahead, making some types of victory practically impossible.
Because you can quickly build bunkers and wait for them in the cities. And early on there is no diversity of troops. Meaning no surprise for you.
Yes they can build their first helicopter gunship... So what, you have starting ASF and you can build the second.
They can build more recons but still need to melee you in your cities with bunkers. They can build some armor...you can do it too. Arty...that's long run you can see it coming.
And they can build strike fighters but that's not so early and you can clearly see airfield lvl2 and react.
If there is an early attack and you win, well that means you can take enemy cities.
That's the best reward.
It is easy to deal with one enemy, the problem would be second and especially third if they are in coalition and decide to attack very early.
Back to the topic, unit experience would be great and I would add daily unit supply on the field as desired. But they probably need to create new game from scratch to accomplish those.