There's definitely a lot that could be done.
I'd imagine some champions would have tried to create their own cities. Maybe defeating that city will yield a city plus a champion. Maybe defeating them will only kill them. Maybe they would want diplomatic capital to join your cause.
Other champions would be real mercenaries, out in the wilds killing monsters with a band of other units. They'd want gold or other resources, and would be gildar and item poor but great fighters, unlocking large parts of the map that they've seen in their travels.
Others might be reclusive mages, locked away in a tower, and want crystal or mana to join your cause. They might bring lost technologies if you hire them on.
I'd imagine some of them hold a grudge, and would join your cause if you declared war on their hated enemies.
Real assassin heroes should be nearly stealthed, and you'd only stumble onto them by accident, or they approach you.
The champions with the godly artifacts should be acting godly. They should be destroying kingdoms, defeating the very elemental lords of land, building many cities, claiming and razing resources, and otherwise act as a force of nature that should be avoided, negotiated with, or killed.
There's so many more options than just sticking champions by themselves with legendary artifacts, asking for a few hundred gildar. If they do stick with nothing fancy like the situations everyone has described above, then just go simple and let champions be recruited through an inn. Makes it much less random, and less easy to exploit, and you can add in different factors like kingdoms with much diplomatic capital tend to attract better heroes. Master of Orion 2 had a good system like that.
