Big Slime:
I created it because I thought that it would be nice to be able to split a slime without killing it, like with gemmation.

Skeletal Dragon:
I wanted a Skeletal Dragon (like in Wesnoth with EoHS) that powers by the death creatures (like some kind of dark ritual). Bone Pile (Necromancer's ability) also gains value on death, but Skeletal Dragon only on enemy death to avoid using summing magic. The value thing makes that a Nullmage can dispel the Skeletal Dragon magic that sustains it.

"skeletons for instance can grow from reaching the promotion line or being upgraded by necro and they have longer range than most other minions" Being able to by upgraded by Necromancer would require to modify Necromancer, and before v0.54 Skeletons didn't grow on promotion line...

Viking:
I created my Viking on Tuesday 10 of January of 2017 (before Pride I think). Originally, the value was tranfere on death to nowhere (just to your morale count) representing that the Viking goes to Valhalla and so none of your army "becomes sad" (lose morale) by the lost. But Both Vampire and Pride transfer morale to ally king so I decided to change that before publishing.

Time Mage:
The coincidence has already been explained in another comment.

Of course, If there is a unit already in the target's starting square you can't use the ability.
The ability itself is not too much powerful (FE simply kills the target), maybe the cost could be increased as you said. What do you think about 7/9/11/12 or 7/10/13/14?
Remember that this unit has only kingmoves, so if you want to expand as much as possible your passive you would need the help of other units.

Sidenote: did you know that the ability can be used to teleport allies as well? Yes... the price is too low, I have to increase it.