Since December 2021, PocketMine-MP has continuously updating their major API version annually, starting from version 4. They've just released version 5 earlier this month and they're planning to release version 6 exactly one year after. This move has required plugin developers to keep up with the API changes, which unfortunately some can't due to some reasons. As of writing this post, a lot of PocketMine-MP 4 plugins on Poggit hasn't been updated to API 5 thus they can't be installed on 1.20 Bedrock servers which are already using version 5. Due to the inability to upload custom plugins on your own to Aternos servers, a lot of Aternos servers might get left behind as they have to stick on using version 4 which only supports Bedrock versions of up to 1.19 to continue using the plugins they need. Not all server administrators are capable of waiting this long just for their plugins to get updated to API 5 by the developers, therefore the current ecosystem is quite disturbing for some server administrators who requires to keep up with Bedrock version updates as soon as possible as well as stable API versions for several reasons.
Some of these server administrators, such as me, are still quite new with their Bedrock servers, therefore using Aternos should be suitable enough for their needs, performance-wise and feature-wise (i.e. using paid hosts are not a must for them). They're just limited to the drawbacks of the ecosystem of PocketMine-MP itself. Therefore, giving other software alternatives similar to what Aternos has done for Java servers should be the best option to let these administrators to keep using Aternos as their starter Bedrock host, and potentially upgrading to Exaroton if they're satisfied with the user experience while using Aternos. After some research, I found the best non-vanilla Bedrock software alternative for PocketMine-MP for these kind of server administrators would be NukkitX / PowerNukkitX. Similar to most Java server software, they're running with Java instead of PHP, which in some ways can be more efficient. Unlike PocketMine-MP, they also feature a stable API which would fit these kind of server administrators' preferences. A plugin I made over 2 years ago still runs perfectly today on both of these software without any code changes at all. Besides that, they also feature an official site of NukkitX plugins managed by their official community, and these plugins would also work perfectly on PowerNukkitX as well. You might wonder what are the differences between NukkitX and PowerNukkitX, well NukkitX is a fork of the original Nukkit project which is no longer maintained as of now with minor improvements. On the other hand, PowerNukkitX is another fork of the NukkitX project which claims to bring a ton of gameplay, performance, feature and other improvements. I personally think which is more suitable for Aternos, either NukkitX or PowerNukkitX greatly depends on the preference of the Aternos team from reading the features and specifications on their official website as well as from several testing to determine which one would run best on the hardware and software of the Aternos systems.
Here are several links which might be useful for the Aternos team:
NukkitX official website: https://cloudburstmc.org/articles/ (The website might show the latest version is Bedrock 1.17.40 but if you check their GitHub repository below they still support the latest Bedrock version)
NukkitX GitHub repository: https://github.com/CloudburstMC/Nukkit
PowerNukkitX GitHub repository: https://github.com/PowerNukkitX/PowerNukkitX
NukkitX / PowerNukkitX official plugins: https://cloudburstmc.org/resources/
(Discord server invite links should be present in their respective website if needed)
In conclusion, for some server administrators, having NukkitX / PowerNukkitX as another Bedrock software option besides PocketMine-MP would be greatly appreciated as they have more stable API as well as more features and optimizations compared to PocketMine-MP. I personally think PocketMine-MP should remain the preferred non-vanilla Bedrock software option, however the existence of NukkitX / PowerNukkitX as another Bedrock software option should boost up the demand of Aternos with the potential of bringing in extra profits and opportunities from the users. I'm really looking forward for a reasonable response from the Aternos team towards this suggestion!