One of the locations that Burn published when he posted the list was prehistoric earth.
If we are going to use location tags in mission, which are trackable behind the scenes but not publicly yet, it may be better to base it on those, but if you want to add in a small planet/moon, it can be based in whatever region you want them to blast off from?
As for the mission text itself:
I approach missions from a 'command bulletin' perspective. Imagine who would be issuing the mission; Is it a security (e.g. Prowl, Soundwave), scientific (Jetfire, Perceptor, Shockwave), combat (Wreckers, Ironhide, Megatron, Onslaught) or medical (Ratchet, First-Aid, ...uh, Shockwave again) mission (or any other you want to invent), and then write out the message they would send.
You need one for each faction
e.g.
the 0-5 mission wrote: 
Moonbuggy Yellow
This is Bumblebee, we've got reports Decepticreeps crawlin' all over that new moon Perceptor has been babbling about. I don't know what they want, or why they are there, but if we leave them to it they'll tear it apart! It's up to us to stop them, and we don't have much time - let's go!!
Moonscream
Reflector has reported a new moon in the alpha sector. You will go there, strip it of all resources, and return them for the Decepticon cause! Do not come back empty handed, or I will feed you to Trypticon!! - Starscream out.
Then you need the win/loss text for each. No one ever reads them, so its probably best you keep them short.
Win:
Well done 'bots, that another planet safe from the Decepticon menace, my plot-armour remains intact.
Loss:
Regroup, Autobots, Megatron's forces were too strong and we were rubbish. I just hope the next time we can do better
but I'm not hopeful.
Win:
Excellent, with these resources I, Starscream can
deliver all the pizza's as that's all I'm good for.
Loss:
You fools are almost as big a losers as I, Starscream.
As for the mission level spread, think about keeping them broad, but not too broad. As a general rule, if there is more than 3 levels difference (e.g. L1-3 works, but L1-4 might not) often you find that the lower levels in it just get demolished and the rewards are quite rare, so it might be best to keep the brackets small for the regular missions and add in the bigger-ranged missions (the L0-5 you suggested) as special events.
(please feel free to disagree, I can only speak from experience.)
Now you just need to think of a cohesive subject for them.
Do you want the lower level missions to be the guards, the mid level to be special ops and the high level to be the assault missions? Work within each bracket accordingly.
Alternately, if you want to have a similar combat mission in each level bracket, think about the canon characters you will use for the command bulletin and what level the would likely be giving order to: (e.g) Wheelie/Starscream at L0-1 → Brawn/Brawl, L2-3ish → Sideswipe/Skywarp at L4-6ish → Ironhide/Shockwave, L7-8 → Prowl/Sixshot L9-10 → Optimus Prime/Megatron L11 or similar.
Once you have that framework, it can be easy to put them together. I've used random G1 examples as an easy reference, but you can alter them to any character set and era you want, as the current canon era encompasses pretty much all generations these days.
Also, I hate Starscream and wanted to trigger the seeker fanboys.
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