Ones to cautious of if you don't want to be too popular/'of the moment':
Erin - currently 40th but is starting to decline in popularity
Violet - currently 78th and has shot up at speed and not peaked yet. I think is set to become huge as people get bored of Lily. 'Ette' / 'Ett' / 'Etta' endings seem to one of the upcoming trends.
Iris - currently 106th. Same as above with Violet though not quite as pronounced.
Scarlett - currently 17th. Doesn't look like its peaked yet and same this as Violet with the ending very on trend.
Isla - 5th
Eve - Together with variations of Ava and Evelyn etc, one of the most popular names about. Will be forever spelling or correcting.
Mia - 10th but has probably peaked.
Grace - 13th though definitely past its peak now.
Olive - Not hugely popular at 233rd, but you'll forever be correcting from Olivia.
As someone else has pointed out, if you want something that sounds strong, avoid y, ia, a, ie, elle or ella endings. They tend to be regarded as very feminine or princessy and are massively popular. Though Cora is possibly one of the few which is the exception to the rule.
I do think 'ette', 'ett', 'etta' are strong sounding but I think this is being picked up, so be wary of anything along those lines. Etta in its own right is going to be huge.
Names that start with A or E are generally all the rage, though some you can get away with.
Cleo and Juno are both on trend with -o endings but are not hugely popular currently at 358th and 626th respectively. I expect they will shoot up the charts but won't become too massive however the danger is with them is that may date badly.
'Grandma' vintage names like Doris, Edith, Nora, Ada, Martha etc which were popular 100 years ago are general pretty strong sounding and very fashionable but still a little marmity which stops them becoming too popular. (These are the ones that tend to break the a ending rule that makes them too wet sounding. Avoid like the plague though). Again they will date too though.
K names are getting bad press at the moment and commonly aren't thought of as classy. The real exceptions are the classics Kate and Katie (I'd even spell Katherine/Kathryn with a C at the moment for that reason).
If you can get your head around all that, you'll be fairly safe in coming up with something not to popular and strong sounding. It sounds like it rules out a lot, but it does leave a lot too plus it should help you work out when and if you want to break 'the rules' too.