The question about Burning was already answered, it is basically a poison counter. Except in red.
(Thus basically "soft forcing" a trio, an in my opinion rather innovative alternative to "hard forcing" it)
My opinion to your other point will hopefully be found in my answer to OldTrees. If it is not to your satisfaction, let me know and I'll go even more into detail.
1) Lava Golem (without 
) is a 5|1 for 5 
. This is barely weaker than a 7|1 Rebirth for 7 .
I can't agree here at all. 5/1 for 5
and 7/1 for 7
is about the same. (6 total stats for 5
is slightly more cost efficient than 8 total stats for 7, but the true formula is Creature strength divided by quantum cost
plus the fixed costs of having to sacrifice a card slot for it, which gives bigger creatures an advantage)
...but the Phoenix ability is ridicoulusly strong and rules this CC & PC heavy meta, which can be easily observed in the BL, for example. Quite surprised by that statement.
2) There are few forced duo creatures (Parasite unupped, Firefly Queen, Graviton Fire Eater, Lycanthrope, Fallen Druid upped, Schrödinger's Cat, Chrysaora, Steam Machine, Forest Spirit, Phase Salvager). However that brings us to the next point.
3) A forced combo is not a problem unless there are very few variations in creating the combo. Off element quanta can be obtained by Quantum Pillars, Nova, both Pendulums, a Pillar or a Mark. While a forced duo design increases the game variety less than an optional duo, it is not restrained to the point of being a problematic forced combo. If Burning Treant is a forced combo rather than an optional combo it will have only 2 options (Pendulum and Pillar). This is ignoring the detail that Burning Treant is not very usable with only 2 elements.
That was the explanation about what people mean when they say "forced combo" however I feel it is more beneficial to teach the reason behind the concept as well.
Increasing game variety tends to result in a more valuable game that is played longer. Thus cards are judged on their ability to improve the game by this metric. Imbalance either decreases(OP) or does not increase(UP) game variety. Cards with more options have an advantage when measuring by this metric. Cards that require specific other cards in order to function fit into fewer decks and thus are at a disadvantage when measuring by this metric. The more restrictive the required combo, the greater the relative variety disadvantage. A novel resulting effect can partially overcome this disadvantage (animate weapon). However the more restrictive the combo the more novel the effect has to be to overcome the disadvantage.
This is all very true. By this definition, Burning Treant is a forced combo (There might only be two decks that can be built about BT, one with Fractal (and maybe without Water Pillars/Pends) and one that contains 6-7 Water Pends and Life Pends respectively and thus doesn't leave much room for individualisation)
What I don't agree with is that this can be changed without giving up the original idea of this card. This can't have those effects (Even in some altered way), but be decent in a mono Fire. (Unless you use standards I can't agree with, which brings us back to 1) )
I think I'll let it be like this and be honest about the disadvantages:
Advantage: Innovative (hopefully), fuels a certain trio.
Disadvantage: Only useable with a very limited possibily for variation.
I do want to point out that this disadvantage counts for quite some cards that are in the game at the moment though and that noone seems to dislike.
While this is interesting I do not find it a frightening example. It would take 54 
for the 9 creature OTK. That is well within the norm.
That sounds reasonable in deed. Agreed.
Based on the amount of space we have for text, I think it should be limited to 2 optional effects.
I also agree.
