I think SimplePlanes as it is now is quite awesome, but I'd like to see something more from you. I know, you're a relatively small dev team, but hey! this is why outsourcers exist! Don't be shy to ask for their help. A very well known company (Wargaming, to be more exact) is outsourcing all of it's HD tank models (previously, the models were quite bad looking, kinda like humans in the infancy of 3D games). I'm sure there are enough guys out there which would surely volunteer and lend you a hand of help.

OK, enough with that long talk. Now, here are my suggestions (and I hope that we'll see a few of 'em added in the next Beta 1.6 update :D )

1. airConsumption XML attribute to engines. This pretty much dictates how much air an engine consumes (in cubic meters) for producing thrust.
2. Advanced building mode. Kinda like 'if Fine-Tuner Mod and SP had a good night' thingy.
3. Unguided rockets that don't need a rocket pod. Self-explanatory to be honest.
4. More colors! The 15 colors we're given to paint our aircraft/boat/whatever is just not enough.
5. Decal layer. It would be very cool if you would add a decal layer to aircrafts. Basically, you can apply a .png file directly onto the aircraft (kinda like in War Thunder). Or you could add a camo layer that uses .dds files
6. Custom lengths. Self-explanatory.
7. explosionRadius and blastPower XML attributes for bombs.
8. Better graphics. Really now, I know you're trying to keep the game as potato-friendly as possible, but the guys that have decent computers are left in the dust in this DERP-art-ment. Besides the reflections and the FXAA 3, we've nothing more. It would be very cool if you'd add god rays, detailed surroundings (more foliage and trees) and, of course, detailed textures.
9. Tank parts. I think this is something that we're all waiting for. You could add customizable tank hulls, guns (which would range from 7.92 mm to 240 or 380 mm in caliber and up to L/100 in length), tracks and turrets! It would be totally awesome, in my opinion, if you would add this.
10. Lastly (or maybe not?.... kidding, don't worry ;) ), you should optimize the game even more than it already is. Yes, the game can run at 60 fps on literally any computer or phone or whatever, but, considering the addition of better graphics, you should really do this.
11. Did you think that was the last thing? Hell no!
12. Was that a suggestion? HELL YEAH!
13. Unlucky number, innit? OK, seriously, you should add gigantic terrain features. In this aspect, you should learn from Minecraft. For the beggining, you could add different biomes with specific thingies, maybe some massive terrains features like gigantic mountains that actually have snow on them and last, but not least, more foliage. Right now, as it is, you can barely spot a tree or pinch of grass on the whole map. C'mon now, I know you can add a superb-looking forest which would make great for tank battles. Or maybe some fjords for these battleships that we can build.
14. This number is actually my age. Or maybe it isn't?? We'll never know. Onto the suggestion: ADD MULTIPLAYER!! Self-explanatory, again. I know it's gonna be a pain in the arse to create a fully-working damage/debris sync model, but I know you can do it. If HellFireKoder and UnstableOrbit are doing it, then you can totally do the same.