I think it would be pretty fun to have a Raid Arena set-up similar to the challenge events. There would be a Rare, Epic, and Legendary Arena. It would have the same mechanics as a normal raid but with special rules same as in the challenge events.
Rare - only 3* heroes and 1 or 2* troops can be used. Set up a 3* defense team and challenge others with your own 3* teams.
Epic - 3* or 4* heroes can be used and 1, 2, or 3* troops can be used.
Legendary - Any heroes can be used and any star troops can be used.
Each arena will have it’s own ranking system and maybe even a little reward like a random world/titan/raid flask or something every once in awhile for being top 10,000 or 1,000 or whatever. I think this will add more incentive to having a strong 3* and 4* roster as well as providing a more level playing field for everyone. It would most certainly be challenging and would truly test the skill and mettle of top players and new players. It will let you use your 3* heroes more instead of just as clean-up in war or for challenge events.