A player can increase the stats of Holos and dark holos with the same kind of Illuvial. On top of that to distinct between holos and dark holos I would suggest a diminishing return mechanism, which gives an advantage to dark holos over holos.
upgrading gets more expensive the better the Illuvial is:
dark holos:
290 --> 300 needs 15 Illuvials of the same kind (normal finish or better)
280 --> 290 needs 14 Illuvials
270 --> 280 needs 13 Illuvials
260 --> 270 needs 12
250 --> 260 needs 11
240 --> 250 needs 10
230 --> 240 needs 9
220 --> 230 needs 8
210 --> 220 needs 7
200 --> 210 needs 6
190 --> 200 needs 5
180 --> 190 needs 4
170 --> 180 needs 3
160 --> 170 And all below needs 2
holos:
290 --> 300 needs 20 Illuvials of the same kind (normal finish or better)
280 --> 290 needs 19 Illuvials
270 --> 280 needs 18 Illuvials
260 --> 270 needs 17
250 --> 260 needs 16
240 --> 250 needs 15
230 --> 240 needs 14
220 --> 230 needs 13
210 --> 220 needs 12
200 --> 210 needs 11
190 --> 200 needs 10
180 --> 190 needs 5
170 --> 180 needs 3
160 --> 170 And all below needs 2
Numbers can ofc. get changed, but with that we would get utility for holos/dark holos and we could see them in Leviathan. I guess people would be happy to find a holo/dark holo with a mechanism like this and normal good stat Illuvials would still keep a lot of their value, because upgrading a holo is a lot of work. This should work decently with the planned pack opening mechanism as well.
We could even create lore around this that gives a reason why holos/dark holos are able to absorb other Illuvials.