Are you referring to coming up with original usernames (as opposed to generating random alphanumeric strings or something like [RandomFirstName]+[RandomLastName]+[RandomBirthYear]? Not actually registering accounts; developing lists of unique usernames for others to register?
I posted this a while ago, which is an easy method to scrape existing/real usernames and to then spin them with a brief script (also included) to make the name original/registerable:
https://www.blackhatworld.com/seo/scraping-real-usernames-for-dummies-extremely-easy-method.948441/
Works pretty much perfectly in terms of having a wide variety of real usernames.
Edit - Just for fun, here's 100 usernames generated from that exact script. Pulled from here:
https://www.reddit.com/r/all/new/
Excluding potentially a few names that happen to already be registered by random chance, these should all be available for registration.
swarewithme
robface13
snarlsoncharge
NoNewzNetwork
RamodRahim
acivvodul
TheNerdWunder
sexyraven11
shazawuel89
Cl0udreigens
Grap3JellyGamer
G8gIe
RangestBootz
DotKyleNg
itsbryandudi
S3venTen01
plantt421
RibalgKaufmann
Waoie
engposition
JayyWarc009
trebeln
Rio_Diemond_YT
zhoshanagallant
SIutty_Spider-Girl
metostrite
Catcwitdonnie
PriestJosephPFerrell
evveryWerr
The3goRaptor
gheleonn
pydelke
bicIennon
TheRealSlimJocer
mitho88o
H_G_BeIls
kdtlrado
wigglyguff2
blackten295
station_lagon
flawegold
Mak0Hika
zdtra
lawnmowertrafflc
Lezter2031
Zeedener
ZaneHoImes
ghebee362
metalmonnzter
aditisharma2027
jackudyssey
EndyIbanez
Xbocks9600
stidenourt
sheriyam
StiamKiwi
kaban-khan
AsheDasha98
Rapt0r_Black
Nugairra
Geralt_uf_Rivia-
ch0wdrister
vasketsinthecorner
Soaridw0w
FoTomArrow
anhay1507502
AugoNewspaperAdmin
Iightmacnuggets
Gh3Dux
1wanttodie1756
GhrowawayLeChat
RTBot
Typesalog
loldiduknow
ratt140
ezbang270
WeaponsWist
pyulcd
GraysonTooles
IciCreamBeavers
riyyuzaky
RPBog
DarkSigeOfHere
AlphaWulfPhilly
fruzenelf
jekbrtz
matbencow
cadre-78
pugtyapp
GeorgeMacDonaId
the_luxiu
GallowPlac3holder
arbyd
rinsl3tte
Thefoarthchosen
Ilick3dthecinnabar
brisgleworm
josethwillis457
PuorMansRollsRoyce
EddiPi
