What was Incursio's trump card ability?
I do not think that the adaption and evolution can be counted as a trump card.

Before Tatsumi, Bullat and Gensei also were users of the Teigu Incursio.
Both Bullat and Gensei did not unlock the capability of evolution in the Teigu like Tatsumi did. This might be due to them not being skilled enough to or just because Tatsumi has so much hidden potential. 

With that said for most Incursio users the invisibility most likely will be the actual trump card, with the only life losing risk being


  As noted, a weakness of the invisibility is that while it masks the user visually from the naked eye, it does not hide their "presence", allowing skilled opponents to sense the user despite the invisibility.


The fact that Tatsumi did manage to awaken this power from the Teigu, most likely is due to his hidden potentional, which seems to get stated by allot of characters through out the series as well


  Tatsumi's name means "Sleeping Dragon", which seems to be a reference to his hidden potential, which has been noted by many different characters in the series. The first creature he was shown fighting was an Earth Dragon, and he later obtained the "Demon Dragon" Teigu.