I'm loving this thread as I am facing a similar issue --
DD2 (18 months, well off the charts for height) is on the verge of outgrowing her CCO by height (she's 34.5" and is riiiight at the 1" limit now). We travel by plane at least 2x per month, sometimes more. The CCO has been amazing -- sad to have reached the end of that era!
DD1 is 5 now and has been fine in the airplane seatbelt since about age 4. Before that, we either took the Radian onboard and FFed it (foot thingy in the luggage) or used a CARES harness. Her travel car seat is a Safety 1st Go Hybrid, easily stowed in the bin or under the seat in front. She also rides in a RSTV for trips that are not car-intensive at our destination (she hates it for frequent car use). The Radian was a total risk, though -- we had several issues of incompatibility with rental cars and ended up FF more than once. She was over 3 when we traveled with that seat, though, so not awful if we had to occasionally FF.
But I'm surprised that nobody has mentioned the True Fit -- it actually makes a phenomenal travel car seat, despite its size. It fits well onto a stroller for schlepping, or you can do the traveling toddler thing (pic below). It installs remarkably well in SO many vehicles -- not only with LATCH but with the seatbelt (the belt lockoffs are really handy). It works in small cars, too, since you can put it very upright even with the headrest attached. We've put it in several dozen rental cars and have never had an issue. Downside: it's not light, and it can be a bit tricky to carry down the aisle, since you might have to lift it above the seatbacks (narrow-body planes mostly). But with its lower profile, I like how it fits on planes far better than Britax convertibles (mind you, I haven't tried the new models with the lower bases). And it can be found for under $150, so if you want a seat primarily for travel, it's not obscene.
PS -- we tried traveling with the Scenera a couple of times and had so many install problems on the destination end in unknown cars -- even with noodles -- that we gave up. We have a CA that we've used in extra cars in the past and dislike how it fit DD1, so we haven't yet tried it with DD2. I hadn't considered it as a travel seat because of what a PITA I found it to be to install. I'll revisit it, since its weight would sure make it a good contender. I'll also think about the RA55 as a possibility, though I don't know that it will take us anywhere near FF age.
A few pics:
True Fit FF on a plane --
True Fit with the Traveling Toddler strap (the purchased one, I'm not that creative to DIY!):
Radian FF on a plane -- the tray table fits over it well: