Hello there,
I have been using Aternos Services 
for the past year and i am still having a fantastic time playing with my friends! Unfortunately, i have realized that Aternos is having specific mods and plugins to pick for your server. So what i really want is a way to add whatever plugin you want with just the link from the bukkit or spigot site(Fluctis Server Hosting is having this feature for 2 years now and its really usefull)! Also, a way to add more modpacks into your server...
I know that there could be security issues if you add FTP Access but if there was a way to add all or the most known mods the FTB and TECHNIC Launcher have, it will be great!
But, if this is not possible think about this:
Make a way for users to upload their mods one by one (just like the custom maps). With this way you will let players create their own modpacks with their favorite mods.Not to mention that you will probably not getting so many threads of adding more and more modpacks anymore!
I hope that my ideas might insire you into some new modifications. 
Thanks for your time! 
P.S. Excuse my english mistakes if i have made some. 