Leliana almost certainly wouldn't be an LI this time around.
If however Bioware went that route, it wouldn't necessarily be 'cheating.' After all 10 years have passed since the events of DA:O and anything could have happened within those ten years. In order for it to be cheating, Bioware would also first have to establish that Leliana and the Warden were still in a relationship.
Well, in the DA2 mision "Faith" where Hawke and Sebastian meet her in the chantry, a romanced Leliana states that the Warden was "dear to her heart", implying that the relationship did last.
But yes, the whole "cheating" thing is precisely the reason why they wouldn't go the LI route with her. Remember the wailing caused by the Zevran bug where he could hook up with Hawke regarless of LI status in DAO?