We don't even have mecha tanks!

Well the thing about avatar is that they have metalbenders. Now everything in our technology today is power>size. Sure they have giant mechs that weigh 3000 tons or mini mechs that are freakishly fast and agile. However, how would they fare against things like nuclear bombs or fighter jets with radar? In Avatar, the entire franchise focuses on bending as their weapons of war. There advanced technology is only possible because of metalbending and firebending/lightning generation. However, they won't ever create things like guns until the extreme far future because as of currently, their warfare style doesn't need guns, it uses bending.

In the next few centuries of avatar, Sci Fi technology will exist (spirit cannons, arc reactor). However, efficient weapons like guns or more precise but less powerful weapons would not be invented due to lack of necessity. (What I'm trying to say is that they'd develop things like nukes before they develop things like guns or snipers even though nukes are more complex and harder to build)

I don't really think they'd truly develop laser guns. What I'd see more would be like the Star Trek phasers (that shoots a beam of energy at various settings) connected to wrist devices, since guns don't exist in this world.

Robotic tanks will still be developed, since that is actually the closest equivalent to guns that their world has. As the tech advances, it'll probably look more like the Iron Man suits from Marvel. Honestly, you could probably expect to have a robotic villain similar to Skynet, Ultron, or Brainiac (to name a few among many options) for the next Avatar to face.

To combine with that, you'll probably see something like lightsabers or other types of energy-based weapons to keep up with high tech but in melee terms in a similar fashion to the Lieutenant's outfit and the Equalists' electric gloves.

To add to that, I'd expect airplanes would get better, but again I would argue that the weapons would be more like putting Star Trek weapons you see on ships like the Enterprise but on more modern types of aircraft (like zeppelins and fighter jets).

Yes, I'd expect to see missiles eventually, but not really so soon. Technologynhas to advance accordingly, meaning we'd have to have rocket launchers first.