Elastic search has functionality called Significant Terms Aggregation.
A significant terms aggregation elasticsearch result could be used to automatically return significant tag keywords. Use possibilities;
The trick behind the significant terms aggregation is in spotting terms that are significantly more common in a result set than they are in the general background of data from which they are drawn.
- Present tag suggestions to users
- Nightly cron job to automatically apply tags to threads added since last run
- ACP option to scan threads added/changed in last x hrs/day and suggest tags to apply to threads, allowing admin to tick a box for them to be applied.