-> Tier III (has no reason to use Wisp now, but very useful as a bulky Magic Bouncer due to the effectiveness of SR)
Foul Play and Knock Off, its best STABs, are now woefully underpowered, and it already had problems getting worn down
before it was auto-burnt. It can't threaten any relevant physical attackers, and is honestly probably a waste of a mega evolution. I'd sooner run Mega Diancie if I really needed that Magic Bounce, because it can actually deal meaningful damage to Stealth Rock users.
-> Tier III (can run Poision Heal set w/ Heal Bell support)
It
can, but I'd much sooner run Gliscor because of its better bulk, speed, and Ground STAB. Breloom just doesn't get the opportunities it needs to set up and gets incinerated by Fire-types, not to mention the blimp.
-> Tier III (same reason as Ttar, but instead of having decent SpA, you have Slack Off)
Is there any point in running a sand setter this pathologically passive? Anything not actively weak to Ground will have a field day with it. I'd just run Tyranitar if I wanted sand that badly.
-> Tier
II III (aside from being worn down easier, the only change is less damage on EQ which can subbed out for Earth Power)
EDIT: I realized it also loses the niche of spreading burns with Scald -- still useful as an Unaware wall but I'll put it in Tier II instead
If you look at the calcs, it doesn't actually wall any of the usable physical attackers.
252+ Atk Guts Conkeldurr Drain Punch vs. 252 HP / 252+ Def Quagsire: 171-202 (43.4 - 51.2%) -- 62.9% chance to 2HKO after Leftovers recovery and burn damage
252+ Atk Pixilate Mega Altaria Facade (140 BP) vs. 252 HP / 252+ Def Quagsire: 232-274 (58.8 - 69.5%) -- guaranteed 2HKO after Leftovers recovery and burn damage
252 Atk Guts Heracross Megahorn vs. 252 HP / 252+ Def Quagsire: 229-270 (58.1 - 68.5%) -- guaranteed 2HKO after Leftovers recovery and burn damage
252 Atk Tough Claws Mega Charizard X Flare Blitz vs. 252 HP / 252+ Def Quagsire in Sun: 153-180 (38.8 - 45.6%) -- 18.4% chance to 2HKO after Stealth Rock, Leftovers recovery, and burn damage <<< Groudon provides sun support, sets rock, switches into Rock- and Ground- types, good stuff.
Basically, being burned all the time lets Quagsire's mediocre bulk get the best of it and make it difficult-to-impossible to keep it at the high % it needs to actually switch into things.
If you want an Unaware wall, Clefable will probably do a better job of it by switching into stuff like Heracross and Conkeldurr and retaliating with Moonblast, while also just having a better offensive presence in the first place (and carrying Heal Bell for Gliscor stall). It's the physically bulkiest Water/Ground type (besides Swampert), but it loses in every other department to Gastrodon or Seismitoad, and Heatran can usually pivot into Fire-type attacks.
-> Tier III (runs a Nasty Plot + Vacuum Wave set in standard)
So like, a shitty Mega Lucario?
+2 252 SpA Adaptability Mega Lucario Vacuum Wave vs. 0 HP / 0 SpD Jirachi: 184-218 (53.9 - 63.9%) -- guaranteed 2HKO
+2 252 SpA Life Orb Toxicroak Vacuum Wave vs. 0 HP / 0 SpD Jirachi: 130-153 (38.1 - 44.8%) -- guaranteed 3HKO