i only really know a lot about the fr3.5 and fr2.0 eurocup series, so you can have my opinion on those
kevin korjus and jean eric vergne are both total no brainers. if gravity can nurture korjus' talent and not push him through to f1 too quickly he's honestly world champion material. he just climbs in something and is quick - witness his wins this year in fr3.5 in his debut season. i'm a massive jev fangirl, but i do think he's better than ricciardo. he's a very articulate guy when he's talking about things he can talk about rather than red bull decided things, and his english is superb. he's also ridiculously quick (duh) and has been the only driver to win both races in a weekend so far this season.
i agree with mitch evans, he seems to be promising and he's got the best mentor possible in mark webber.
robert wickens is quick, but he hasn't been as consistent as vergne in the same team. he's also lost quite a few points due to errors which you'd hope his experience would have dealt with by now. if he's fr3.5 champion by the end of the year i'm willing to eat my hat though!
alexander rossi i think needs a good consistent season. he's had a couple of big mistakes this year (monaco, and he got involved in some mid-field shenanigans in both races at the nurburgring), but when he's on it he's really good. he's also very articulate and would be a great ambassador for the usa in formula one.
looking a bit further down (so maybe 2015-16 in f1, if it still exists) there's carlos sainz junior in the formula renault eurocup with red bull. both him and daniil kvyat have been pretty good this year in a HUGE field with some seriously competitive opposition. they're very young though, which gives red bull about 3-4 years between jev and them. away from red bull in the same series there's robin frijns who's leading the series at the moment and third in the nec series too. gravity have a young spanish lad called javier tarancon who seems a bit inconsistent so far but on his day is really quick. there's 3 or 4 formul'academy/f4 eurocup graduates from last year being pretty competitive - paul loup chatin, mathieu jaminet and norman nato are french and stoffel vandoorne who's belgian. they're all in their second year of car racing and they're all right up there. all 3 have had a podium this year and chatin is in with a chance of winning the fr2.0 alps title if he plays his cards right.
in terms of up-and-coming british drivers i'm not really sure who to suggest. alexander sims has always been hyped up and marked for success but as yet he hasn't been picked up by a f1 team? lewis williamson did an excellent job in fr3.5 at motorland when he stepped in for ricciardo and he's doing well in gp3. there's a british-singapore kid called melville mckee in the eurocup and the alps series who seems to be producing some pretty awesome pace too, i think it's his first season of cars. he's actually 4th in the alps series which is seriously good.