Hello IceFir3, welcome to the forum!
Most of your suggestions seem to be reasonable to me. Let's have a look at them:
Configurable amount of time in which the amoeba (BD style and amoeba with content) turns into boulders
This one absolutely makes sense, and shouldn't be hard to add.
Bombs that can bounce off elements until they can't anymore and explode. It would explode when it lands onto a non-slippery element, or when it cannot bounce off more.
For this feature, I would suggest using a new element with different graphics would be required, as it would be rather unexpected to have normal looking bombs that do not explode when falling on a slippery wall. Besides this, I like the idea!
Rocks that can't fall down from an element despite such element being slippery. Or at least a CE that allows the CE to override the slippery value of the element below.
This sounds a bit like the existing "stable DX bomb"... Question: Would that rock fall down other rocks? If yes, I would find it a bit strange.
Allow gems to pass through quicksand. (default off)
This sounds like a reasonable idea to me.
Allow a CE to explode in a 5x5 fashion.
Same here. Would then also need a 5x5 area to define its content.
Making CEs that behave like those elements described above is really hard and requires lots of trickery.
There are ideas for "new elements" every now and then where I think they could easily be constructed using CEs, but all the ideas above are not (or hard to do, as you said), so I will have a look what could be added to the R'n'D game engine.
The ideas come from a really old game called Rocks'N'Gems, a Net Yaroze Boulderdash clone.
Yes, I've heard of it (but never played it) -- the name "Rocks'N'Gems" always sounded a bit "inspired" by the name "Rocks'n'Diamonds" to me...
No new elements are needed, though.
Well, probably it could all be done by adding new CE properties, but some of them seem to be better done by just adding new elements with these properties -- especially the "bouncing bombs" and "not falling down from slippery ground" properties seem to be hard to add to CEs in a generic way, as they are a bit special, I think.