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Single mods can already be installed: https://aternos.org/software/mods

If you talk about multiple mods:

Quote from Roman

We still have that on our ideas list, but we discussed the "multiple mods" topic last week in our team meeting.

For now it doesn't make really sense.
If we add a feature, we want to keep it simple, understandable and usable for everyone. The work and time we would invest into adding this is too huge for the small amount of users who would use it.

Our current statistics show that modpacks and mods are only used by a small amount of people. Keeping all that stuff up to date is already a huge work where only some people benefit from.

Plugin & Mods together:

Quote from Roman

To run plugins in modpacks a special server software ist required. Sponge or Cauldron for example.
Cauldron has been discontinued and Sponge is not released yet/still in beta.
Also we only add mod packs in their official configuration as they are distributed by the author. That means that we don't change the server version to e. G. Cauldron. This would decrease the stability of the mod packs.

We already have some modpacks which run with Cauldron out of the box.