Simple and good plugin, thank you for this.
Other possible extensions which can be added:
1) Possibility to give experience to villagers through trades for increase their level.
2) Possibility trade custom items e.g. 15 coals for 2 emeralds or for one sword.
3) Possibility change all possible villager trades individually e.g. for Armorer - Journeyman - 1 Lavabucket for 5 Emeralds
4) Possibility add or remove items in random trade groups
see
https://minecraft.gamepedia.com/Trading
5) Modified prices can be affected by events or your actions