I think the best way to create new mechanics is to experiment with arena design. We've been seeing one of these 'variable' arenas for one fight each tier lately, although it would be more interesting if tank positioning/movement played an active part.

Visual tells are another way to make fights more interesting, especially if you remove boss cast bars as well.

I personally really like creative mechanics around undeath. Forgall from Mhach had a nice Necropurge/Megadeath mechanic, and Neo Exdeath likewise had a Living Dead styled mechanic in Beyond Death. Speaking of Neo Exdeath, it's much more interesting to integrate tankbusters alongside other mechanics rather than to have them go out in isolation when everyone else is in downtime.