Turning into a collection of assets is usually what happens with such packs over time.
Its also one of the most important roles it can fulfill.
I don't know what you want (in terms of "what direction", not in terms of "i don't care"),
so my recommendation is about ease of use and simplicity.
-Giving credit to the Terrain Pack is a 100% no-argue clause.
You´re using it, you´re giving credits, its that easy.
For simplicity I´d recommend giving credits to the Terrain Pack as a whole serves the purpose,
the single contributors are credited in the Terrain Pack itself for their respective work.
-Contributing assets may be an argue point.
For the sake of the modding community, people using the pack and creating assets should contribute,
but its hard to regulate. I see no reason not to contribute your assets to the pack.
However, if someone really does not want to, should they be forced to share all their assets for using one single part of Terrain Pack?
I think not. Recommendation: For every asset you use from Terrain Pack, you have to contribute an asset from your mod, if you created some.
My hope would be that people are too lazy to count and just contribute all.
(Alternatively take a donation for using it without contribution, but gl sharing that fair with all Terain Pack contributors, so rather not)
-Donations definately are an argue point.
If you want donations, make a site and ask modders to donate. (usually its gonna be disappointing...)
But regulating donations for mods using Terrain Pack is a no-go. (by either saying they can't take donations or have to give 5% or so)
Its insanely hard to regulate in a fair way anyway.
By forbidding donations you`d say because you worked for free everyone else has to work for free too.
By taking a certain percentage you´re assigning a fixed value of the mod to the Terrain Pack.
Even the edge case, lets say someone creates a mod with 0 additional work just by including Terrain Pack and gets a ****ton of donations for it,
their main ability is "being good at getting donations", you have no claim on that either.
(From a point of fairness of course you should get a lot of that)
Basically I´m saying the amount of donations is not directly tied to amount/quality of work, but also marketing.
If someone were to run an actual business from a product including Terrain Pack thats a completely different case.
But from what I´ve seen any modder is very far from making actual revenue compared to their work time, its more like pocket money.
Taking donations for a mod is out of scope of Terrain Pack, so any interference on that would be ill advised.
However, asking the modders and fans of the mod to donate to Terrain Pack is completely valid and fair.