Hi there,
Two things came to my mind
-
If there are two (or more) crystals of the same colour, than the tiles could do double (multiple) damage. It does happen that you have two crystals of the same colour, but than you are dissapointed because you lose them both at the same time and no benefit from it. Double damage would be nice feature.
-
The damage or regeneration of HP per round should be cumulative, at least if it comes from the same hero. It is kind of different from other buffs to me - simillar to minions. Let me give you an example (my case) I have a Gravemaker with very fast mana gain. The basic damage of his special is nothing special but than you have quite high damage per two rounds. But it happes that you charge him just the other round after he used special skill… Than the main advange (very fast mana gain and damage per round) is more or less lost since you can not use it to its full extend.