1. For each topic, get a list of the distinct users who posted there in the last 24 hours.
2. Give the topic a score equal to the sum of each such user's total earned merit.
3. Sort by topic score descending, and secondarily by the usual last post time.
It will be best especially for altcoin announcements board. Its good to see that you are thinking about it.
The main problem is now altcoin announcements board. I have some suggestion and its no need much technical work.
1. Disable posting on altcoin announcements below Jr. member rank. Keep only read mode for newbie rank.
2. For reduce spam from others board just disable signature below member rank include copper membership. Peoples are skipping merit system and buying copper membership. Let them(newbie) earn from other campaign. So they are continue spamming.
This spam should just be deleted
That's the problem,
just be deleted. Why should not punished them ? Even temporary punishment. Like if a newbie 10 post reported and deleted by moderator, then he should get ban for 3 days. For each rank there will be different deleted post count. Something like that will help reduce more spam.