Yea, stealth units in a tactical battles could be interesting if they are hidden from the enemy's view (and data collection) except for when it is next to an enemy controlled tile (tile with an enemy unit).
However, 2 tiles away from the enemy, still invisible.
It'd also be cool if individual assasins (single unit or Champion) could even BLEND IN with enemy units, being only visible when next to a tile containing an enemy Champion or enemy Sovereign, and also being able to share a tile with enemy units. Such an assassin should also be visible if it were to attack any enemy unit with a melee attack.
Of course, if spotted by an enemy Champion while sharing a tile with an enemy unit ... the assasin suffers some "attacks of opportunity" from those that share his tile.
Therefore, the role of the individual assassin would be to try and isolate enemy Champions and attack them (when they are alone).
