I did a test session with this mod and the Creeper2 extension enabled. Here's what I've noticed (my apologies in lumping these together... I didn't feel like splitting it):

1. Biters are slowed down by the creep, too. This can be used to the player's advantage with turret creep.
2. Possibly from Creeper2, I noticed some areas where creep was moving into water areas. I think it was replacing the mud-water tiles, but not sure. These are walkable tiles with a speed reduction, so I guess that makes sense. Not sure if you want to fix that, or leave it as intentional behavior. I suppose if biters were no longer slowed by creep, it'd be a way for them to "remove" slowdown choke points from in front of them, though once reclaiming the land, it'll result in the shoreline being altered.
3. Some mod started fires didn't light the creep on fire, but they do catch to trees. Not sure if it's the method used to determine if creep catches fire, or if it's the other mod determining what the fire spreads to? Example mods:
   Incendiary Munitions (https://mods.factorio.com/mod/IncendiaryMunitions)
   Napalm Artillery (https://mods.factorio.com/mod/NapalmArtillery)
4. Any flamethrower type weapon that doesn't actually start fires on the ground (tank, mod added vehicles using the same type of weapon) cannot be used to catch the creep on fire. Difficult to say which way would be better. :P
5. Creep burns to and through spawners and worms, killing them (well, most of them). As such, when used in conjunction with a mod that allows fire to hop more times before dying (https://mods.factorio.com/mod/wildfire_season), catching one little corner of the creep on fire can eliminate an entire nest. Don't need turret creep anymore.... (Did result in some very "pretty" sights at night, though, especially when used with a mod that makes nights darker. :) )
6. Burnt out (and I'm assuming collected) creep converts the ground tile under it to something else. Results in "terraforming" (for lack of a better word) the terrain vs returning it to the way it used to be.
7. Creep without any spawners or worms nearby will last forever, meaning your only choice is to burn or collect. Personally, I think it'd be nice if there was some kind of "reverse" spread factor that would cause creep to die off over time if it's not connected/close enough to something to keep it alive.

2 & 5-7 above result in the biters now feeling (more so) like an invading species of the planet. I'm still trying to figure out if I like that. :) I think it would be nice if after burning/collecting creep, even if not immediate, that the original terrain would eventually return.

I do like the ability to keep the creep back using higher tier tiles, though. Gives a good balance. Pro in that you keep the creep back and can protect your defenses and any dragon's teeth, etc, but with a side effect that if you aren't strategic with its placement, biters actually get a speed boost and can rush your defenses faster. :P

1. You must use Rampant and disable "Units affected by tiles". Unfortunately it will make biters run fast in AlienBiomes too, but as biters are natives on this planet I think it is ok. I'll check if without Rampant this bug exists in Vanilla, maybe will add a check and game modification later.

Everything else is not related to Warmonger mod. Creeper2 is just a very quick test porting for Warmonger. I personally do not use Creeper with burn function, evolution production, "accidental" creep creation - it's cheat. I find interesting in it only spread from biters bases.

Concerning fire: Example of more balanced fire is Rampant new enemies inferno spitters.

I described in other thread how I do see balanced Creeper mod. But it requires time to implement. I prefer to focus on Warmonger features - they are more interesting.

Please give feedback for Warmonger balance. It is important.
Concerning Creeper2 the only interesting is - do players buildings start to corrode when creep spread reaches them. Did you experience it?

You must use Rampant and disable "Units affected by tiles".

Ah, hadn't noticed that setting with Rampant before. I'll take a look, however this means that the mud-water and concrete tiles would no longer affect them, either. Not sure how I feel about that tradeoff. :/ Agreed with your concept on the effects vs Alien Biomes, though.

Everything else is not related to Warmonger mod.

Ok, so the changing of the tiles under the creep after being burnt is purely Creeper2? I didn't do much in actual collecting of creep so didn't notice if the ground tiles returned as normal there. It was a test game for something else with Creative Mod enabled and in full use, and once I finished with the initial test, I went on to test the effects of some other mods, yours included, so I had no need to actually collect the creep for biomass. That said, I did do a few test collections with the collection tool and it seemed to always return 0 biomass? Only times I got biomass was while replacing creep with concrete.

Concerning Creeper2 the only interesting is - do players buildings start to corrode when creep spread reaches them. Did you experience it?

I believe so, yes. Again, this was a test game for something else that I turned into testing the feel (vs functionality) of your mods, but I did at one point have some dragon's teeth that were constantly being repaired by con bots until I threw concrete under them.

I'll take a look, however this means that the mud-water and concrete tiles would no longer affect them, either. Not sure how I feel about that tradeoff. :/

You can post an issue for Wube in their forum. They fix issues sometimes. I believe Veden added the "work around" option after a lot of negative feedback received and game developers not fixing it. Main game issues not always can be fixed by mods, while moderators on factorio forum delete bug reports :-(

Ok, so the changing of the tiles under the creep after being burnt is purely Creeper2?
mmm, 99% yes. Creeper incorrectly removes creep of Krastorio2 (v1.2 100% and maybe even for older K2 - the obsolete one on mod portal available now). Warmonger is based on K2 v.1.2 code.

Creeper2 is a full clone of original Creeper with all its issues. I don't plan to fix them. I only fixed creep generation to test Warmonger. The plan of normal Creeper is in other thread. But I'm implementing creep removing building now (it will replace shovel and deconstruction) and fixing creep balance (decreased obtained biomass by 40% - it requires chunks with biters to be deleted and re-generated with Warmonger 1.2.2 or even 1.2.3 - easier to start new map). Then I plan to add a few late-game creep features. Creeper adoption plan is very far away. Warmonger balance is the king!

That said, I did do a few test collections with the collection tool and it seemed to always return 0 biomass?

Pls read change log for Warmonger. It is by design to make Creeper2 being not a complete cheat-machine. And for future Warmonger challenge features...

Only times I got biomass was while replacing creep with concrete.

Please don't do that. To turn off this cheat I need to completely replace Krastorio2 code. I decided not to do that - who wants a fair play will play without path replacing issue of factorio engine (I don't have a hope Wube will fix it and no time to ping them about it).

dragon's teeth ? It works only for stupid vanilla biters. Rampant biters eat them. That's why Factorio without Rampant is not playable anymore IMHO.

Thank you for Warmonger feedback!

Only times I got biomass was while replacing creep with concrete.

Please don't do that.

Haha, sorry! It wasn't on purpose, actually. I decided at one point to expand the concrete from my wall out further than what I already had to prevent my flamethrower turrets from catching the creep on fire and thus completely burning out the nests that were settling near me. As the creep had already expanded pretty close (and biters dying was leaving patches), this is how I discovered this.

dragon's teeth ? It works only for stupid vanilla biters. Rampant biters eat them.

They were working ok for me. I was using 3x3 dragon's teeth with a + design in one area instead of a full wall. Biters were for the most part ignoring them, though they may have been wailing on them some, but they died quickly enough and then con bots were on scene for any needed repairs and replacements. I also didn't place the dragon's teeth in a perfect grid. It was an off the cuff quick stop gap measure to slow them down for the laser turrets vs placing more turrets, so there may have been some wide gaps in there. XD