Huge. And honestly probably a trap given Thopters. The raw Timmy is nice, but might just want to be an 8/8 trample.

I like the sound of an 8/8 trample.

Playable and a powerful reason to play canisters in a subtle manner

Perhaps a little too subtle, but we'll see.

...what part of play giant green creatures is subtle?

Its the canister interaction with the high green cost that is subtle

Was terrible compared to Eldrazi Devastator

You don't have to compare it too closely with Eldrazi Devastator because this is not an eldrazi set. Part of the goal of an eldrazi set is to give everyone a chance to play a big eldrazi. Our goals are different.

This is just too weak at uncommon. City Sentinel at common was the same but didn't have trample.

Upscale Pelakka Wurm?

If you want to keep it simple you can slap some hexproof on there like a Plated Crusher (again, an example from an eldrazi set)

How do we feel about expensive things that you probably want to use canisters on also making canisters? (Too cyclical? A waste of a token making slot?) I was thinking big, trample, reach, and make some canisters. The trample works with attacking, the reach works with blocking, the canisters let you do both. It also leaves something behind even if your opponent immediately removes it. The number of canisters could be based on how many you sacrificed the turn you cast it but it doesn't have to be.