Tainted Observer

Poison and proliferate have always been the natural pairing: toxic puts the counters down, proliferate multiplies what is already there. What this bird does is fold both halves of that engine into one flyer and gate the second half behind a repeatable trigger. Each additional creature you control lets you buy a proliferate for two mana, which means the proliferate is not a stapled-on keyword doing a fixed amount of work but a tap you can open again and again as the board develops. That reframes the toxic damage. A single poison counter from an evasive 2/3 is not the threat; it is the seed. Every proliferate afterward advances that counter, and it advances everything else with counters in the same breath: planeswalker loyalty, charge counters, whatever the rest of the board is stacking. The design tension is throughput. A go-wide green-blue board can pay the many times a turn, but the trigger only fires on creatures entering, so the ceiling is set by how fast you convert bodies into activations rather than by mana alone. It is a build-around wearing an aggressive body: cast it as a two-power flyer and it chips away, or feed it a stream of creatures and let the proliferate compound the poison clock while quietly ticking up everything else you care about.