I don't play the standard Wesnoth scenarios very often anymore, but because Wesnoth has a full scripting language in it to construct custom scenarios, people have invented whole new games for it.

I personally enjoy the "SurvivalX" family of single-unit multiplayer RPG campaigns: you have a party of adventurers, one unit per player, that gain experience and gold through combat, and can develop skills, spells, abilities, new weapons, and otherwise become more powerful, leading up to large boss fights against the leaders of the opposing armies. And when you lose your one unit, you lose.

RPG scenarios provide a very different feel from the standard game of Wesnoth, but personally I enjoy them far more.