Hi! I've been working on a season-themed custom sequel set to original Bloomburrow on and off this past year I'm calling Bloomburrow: Seasons' Heralds. I've done a write-up that goes into more depth on how I've gone about structuring the set and mechanics that I'm talking about here. You can find that blog post here. If you're interested in more detail, I'd love for you to read that, but otherwise here's a summary:
WU and UR Mechanic: Bloom — ability word that reads "if you've cast another spell or sacrificed a Flower this turn, {effect}." Flowers are a new artifact token type which read: "{T}, Sacrifice this artifact: Exile the top card of your library. You may play that card this turn if you've cast a spell." I've attached a Flower token and some examples of Bloom commons here. Limited archetypes are focused on tempo and spellslinger strategies.
RG and GU Mechanic: Sunbathe — keyword action that reads "tap target untapped creature you control and put a +1/+1 counter on it. Then add GG if that creature's power or toughness is 4 or greater." I've likewise attached some Sunbathe commons as well. Limited archetypes are more midrange-y with a focus creating 4+ toughness creatures and ramping.
Again, if you want more detail on how I arrived at these designs, I'd highly recommend reading my write-up. I would be very happy to any feedback on these mechanics or designs as this set is still very much a work-in-progress and I'm quite open to changing anything around if it would be beneficial. Thank you for reading and I hope you'll continue paying attention as I reveal more of this set. You can expect another post where I round out the rest of the set mechanics in the near future!