What if Mantine got Roost?
Mantine has always wanted recovery, as being weak to SR and a crippling weakness to Electric has prevented it from being the special wall it desperately wanted to be. Roost allows Mantine to effectively stall out many opponents and frustrate many Grass-types due to being neutral to their attacks and having a SE STAB Air Slash. In particular, Mantine would become Ludicolo's best counter. Roost also enables Mantine to shed his Rock weakness and halve his Electric weakness, and Mantine's typing naturally gives him resistances to Fire, Fighting and Bug, making him a solid anti-metagame check with a decent base speed. He'd still get frustrated by Toxic, but no reason to expect perfection.
What if Floatzel got Work Up?
I still don't know why he didn't. There isn't much point for Floatzel to gain Swords Dance now, as Samurott is better at sweeping with it and Ninjask is better at BPing it. On the other hand, with an excellent speed and decent offenses on both ends, Work Up would allow Floatzel to rip through many Pokemon with a combination of its decent offensive moveset - its issue would still be a case of 4MSS - I can see offensive Floatzels running Hydro Pump and Ice Beam, with the fourth move either being Focus Blast or Crunch, both of which have merits. Floatzel would also be able to BP Work Up, giving it a unique niche that can benefit mixed sweepers.