Battle Nations was an amazing game and I'd love to see it remade in all its glory. However, a few things would need to change if the game returned.
First would be the time it takes to do certain activities. Time can be a fun mechanic, something that helps you get a sense of progress for the actions you take, part of the same grinding for resources and the like. However, certain elements of the game were designed to bottle neck a players ability to progress quickly without spending paid currency. Healing/repairing units is the greatest offender of this. After a series of battles using whatever units I still had alive, I'd return to base and realize it was going to take nearly 3 to 4 days to repair my units, if they were continuously being cycled through. It is an interesting mechanic to have your units be injured, but Hospitals and Repair bays should dramatically reduce the time of certain units that need to get through them.
Worker management was another fun mechanic. You had to properly figure out how to best organize your population into certain tasks and effectively far gold or experience. However, turning off a building still trapped some workers inside. In a new version, turning off a building should mean ALL workers are available to be assigned elsewhere. Otherwise your faced with potentially demolishing some buildings just to free up a few spare workers for a building you rarely use.
Another issue to adress would be power creep. Some fan favorite units simply become outclassed latter in the game. I'd propose adding higher possible levels for weaker troups, allowing them to follow you throughout the game. Possible buff and nerfs to some other units might help create strategic niches where an early unit might actually be the better choice. With as large of a unite roster, it'd be tricky, but with some finetuning after release it could be obtainable.
Another introduced mechanic that could be enjoyable would be individual unit experience. Nameless soldier 273 now has a story, he's the unit that has been with you through Gantas, leveld up through the Silver Wolf uprising, and got that last miracle shot off that saved the day. Allowing each unite to gain experience seperately from the rest of the units creates a story. Suddenly healing a certain unite feels more important, and that wait for Soldier 273 feels okay, because you want HIM with you, not just anyone will do.
The final change would be to drop rates. Grinding can be part of the fun of this game, but for certain things, it can feel borderline impossible. The biggest offender are boss drop units. I was one of the first players of Battle Nations, and I ground out some of those bosses every time the respawned. I didn't even know what a "melee fragment" was until right before the game closed, one dropped. Years of playing and i only found it near the end. Thats rediculous. Grinding should feel like doing it almost guarantees your success eventually, without having to beat your head against a wall while doing it.