I'd like a romance with someone of equal rank (I'm fine with being stuck at sergeant or some such myself, thank you very much), but make that romance very subtle and subdued because both parties actually stick to the no fraternisation rules. Just looks and small gestures and the occasional innuendo. At the end of the main story,the protagonist quits military service and the romance's resolution - or its real start, if you will - is in the epilogue.
This is actually a good demonstration of how such a relationship could work. Deferred culmination- in which interest is there but professionalism is the obstacle- is a good use of the dynamic that Bioware woefully passed on.
Good example of the dynamic would be Roy Mustand and Riza Hawkeye from the anime Fullmetal Alchemist.
The problem is that in the end, you'll always end up with characters who will end up with less dialogue, because of the nature of their position in the game, will always have fewer opportunities for as much dialogue as followers. One of the nice things about the characters our PC takes on quests is that they have a lot to say throughout the story, thus you get a better illustration of their personalities than most.
This isn't inherent by any means. EDI, for example, probably had as much or more dialogue in ME2 as any of the companions, romances included, and more 'responsiveness' to the plot twists in the missions. Most of the ME2 companion dialogue was interchangeable filler. Morevoer, dialogue budgets could simply be re-allocated. Companions you take into the field have their 'field' budget and ship conversations, but non-combat NPCs could simply have more non-mission dialogue.
There's also the point that, at least in Mass Effect, romanced companions had very, very few romance-specific dialogues or reflections on mission developments or progress.
There are number of different ways in which someone can express characterization, even along missions, without needing to be a squad-mate/subordinate.
-Voice in the head/distant observer- the EDI/Cortana route, in which the character sees what the protagonist sees and has a communication link. Support character isn't physically present, but can have a significant role in developing the story and context.
-Escorted non-combatant- the person who is there because Reasons, but stays back and doesn't participate in combat. Typically used if the escortee is a specialist with a certain skill set, or being rescued, or simply someone a non-controlled NPC. Used on a number of occasions in Dragon Age.
-Parallel progression- the person is contributing to the mission in a way, is able to coordinate and communicate with the PC, but not on the PC's path and not under their command. Mordin in ME3's Surkesh mission, Nihlus in ME1's tutorial, but most relevantly the entire rest of the squad during the Citadel DLC, where all of Shepard's crew are fighting the mercs from the flanks.
-In-mission link-up. When the level/mission design allows and encourages breaks from the combat to talk-to and coordinate with NPCs. Best example is the Winter Palace in DAI, where the PC can talk with the (potentially romanced) Advisors across the mission. In Mass Effect, an example of an applicable stage setup would be Noveria- where Shepard has numerous 'civilian spaces' (the Port, the huddled survivors) where a non-combat NPC could be present and/or follow as the player leaves and returns.
And that's without the possibility of post-mission conversations to reflect on what happened.
-Imagine if, say, Kelly in ME2, serving as informal counselor if you wanted, could let the player role-play reactions and justifications to their Big Decisions? That'd add another layer of role-playing opportunity to players by not simply accepting the in-the-field dialogue as the protagonists One True Expression of Feeling that would never be revisted again. The NPC could indicate an opinion or view, even as the PC can run the gauntlet from 'I did what I had to' to 'I have doubts' to 'I lied, but it had to be done.'