I stumbled across a fic last week that played with this pairing in a way that stuck with me. It was less about immediate romance and more about the physical reality of their roles—Claire patching up Engel's wounds after a skirmish, the forced proximity of a cramped med-bay, that kind of thing. The tension came from the sheer difference in their demeanors; Claire's methodical, almost detached professionalism versus Engel's... well, Engel-ness. The author didn't have them confess anything, just let the air get thick with unspoken things during a routine debrief.
What worked, honestly, was the restraint. No grand declarations, just Engel leaving a slightly too-long pause after saying 'Thanks, Doc,' and Claire fumbling a bioscan cartridge. It felt earned. The best fics for them seem to build off that inherent dynamic of caretaker and damaged warrior, letting the romance simmer in the gaps between duties.