Bring New Life to Monstrosity
I know development has already stated they have no plans to continue updating monstrosity, however this is all based on the fact that players don't play the content due to it having no reward aside from the fun alone. The perfect and simple fix to this would be to add key items for mastering a species, something akin to the cheers from monster rearing.
Example being getting 99 on the Behemoth species would allow you to get a key item that boosts your trip attack rate by 3%
OR
You can choose an instinct to equip on your actual character and mastering a species unlocks their instincts to be equipped.
This alone would cause a huge flood of people playing the content again, and I think personally the least amount of work for the development team. However if they felt it would be too overpowered then another super simple addition but something that would equally make people want to play monstrosity would be to simply add items that you can trade your infamy for. Just like in ambuscade trading hallmarks for items required in REMA's, allow infamy to be exchanged for these items too. Then you'd have all kinds of people begging for monstrosity updates.
If the development team didn't like either of those ideas, something a bit more involved but would require more effort for the players would be adding some sort of treasure hunt or BCNM type battle for monstrosity, giving endgame drops like chapters, upgrade materials, etc.
Some minor ideas that wouldn't have as much impact on players would be:
-To have a ranking system that would rank the servers top monipulators each conquest tally period rewarding them with items or gil for different rankings.
-A unique Call Beast Jug that would summon the selected species enhanced with equipped instincts OR a trust summon that would summon selected species.
-An item that can be used outside of monstrosity to turn into a costume of selected species.
I know the development team is short staffed and are busy keeping the game updated with current end game, but if you read this please just give this one last chance, add one single key item or allow for trading infamy and you will see just how much this content could thrive. Bring back Monstrosity for the players that loved it and to let players that never played it realize how fun it really is. Thank you for your time and all that you do for our beloved game.