I would like to thank everyone who contributed to this thread, particularly Markmeister and Andreas. With the guidance here, I figured out how to edit world_default for a new fictional league in v12, despite never having worked with .xml before.
In case anyone else is trying, I post mine here. Its a single continent, seven ethnicities, two nations, each nation has two states, each state has four major-league sized cities and five minor league-sized cities.
Code:
<?xml version="1.0" encoding="UTF-8"?>
<WORLD id="1" name="default" fileversion="OOTP Developments 2011-06-29 16:33:09">
<LANGUAGES>
<LANGUAGE id="0" name="Setherian" />
<LANGUAGE id="1" name="Helderean" />
<LANGUAGE id="2" name="Nomystian" />
<LANGUAGE id="3" name="Rebular" />
<LANGUAGE id="4" name="Foel" />
<LANGUAGE id="5" name="Krel" />
<LANGUAGE id="6" name="Amandean" />
</LANGUAGES>
<CONTINENTS>
<CONTINENT id="10" name="Setheridge" pop="14112200" abbr="SETH" dem="Setherian">
<NATIONS>
<NATION id="270" name="Setheridge" pop="7649000" lid="0" gender="0" bbqual="3" abbr="STH" dem="Setherian">
<ETHNICITIES>
<ETHNICITY lid="0" pct="62" />
<ETHNICITY lid="1" pct="2" />
<ETHNICITY lid="2" pct="11" />
<ETHNICITY lid="3" pct="8" />
<ETHNICITY lid="4" pct="7" />
<ETHNICITY lid="5" pct="5" />
<ETHNICITY lid="6" pct="5" />
</ETHNICITIES>
<STATES>
<STATE id="3500" name="Belville Plains" pop="4068000" abbr="BEL">
<CITIES>
<CITY id="95000" name="Binney" pop="974000" lat="120" long="-15" abbr="BIN" />
<CITY id="95001" name="Kendall" pop="1352000" lat="70" long="-135" abbr="KEN" />
<CITY id="95002" name="McKilligan" pop="632000" lat="80" long="-50" abbr="MCK" />
<CITY id="95003" name="Mount Waylandtown" pop="287000" lat="50" long="7" abbr="MTW" />
<CITY id="95004" name="Blue Square" pop="257000" lat="90" long="-105" abbr="BSQ" />
<CITY id="95005" name="Goodwyn Fork" pop="102000" lat="40" long="-110" abbr="GWF" />
<CITY id="95006" name="Red Beach" pop="178000" lat="110" long="5" abbr="RB" />
<CITY id="95007" name="Sandbachana Woods" pop="153000" lat="70" long="-20" abbr="SBW" />
<CITY id="95008" name="Tredgettmond Butte" pop="133000" lat="120" long="-120" abbr="TMB" />
</CITIES>
</STATE>
<STATE id="3501" name="Whistedana" pop="3581000" abbr="WHS">
<CITIES>
<CITY id="95020" name="Boundymount Corner" pop="303000" lat="-5" long="-150" abbr="BMC" />
<CITY id="95021" name="New Ungerapolis" pop="802000" lat="-45" long="-85" abbr="NEW" />
<CITY id="95022" name="Schawmount City" pop="1277000" lat="20" long="0" abbr="SCH" />
<CITY id="95023" name="Tillbrookson Bluff" pop="449000" lat="10" long="-65" abbr="TIL" />
<CITY id="95024" name="Bottom Brook" pop="229000" lat="25" long="-125" abbr="BBR" />
<CITY id="95025" name="Finchamland" pop="198000" lat="15" long="-25" abbr="FIN" />
<CITY id="95026" name="MacKaness" pop="143000" lat="-30" long="-70" abbr="MAC" />
<CITY id="95027" name="Mount Jarviswell" pop="98000" lat="7" long="-60" abbr="MTJ" />
<CITY id="95028" name="Red Mine" pop="82000" lat="-25" long="-90" abbr="RM" />
</CITIES>
</STATE>
</STATES>
</NATION>
<NATION id="271" name="Helderan" pop="6463200" lid="1" gender="0" bbqual="3" abbr="HEL" dem="Helderan">
<ETHNICITIES>
<ETHNICITY lid="0" pct="3" />
<ETHNICITY lid="1" pct="65" />
<ETHNICITY lid="2" pct="1" />
<ETHNICITY lid="3" pct="1" />
<ETHNICITY lid="4" pct="11" />
<ETHNICITY lid="5" pct="2" />
<ETHNICITY lid="6" pct="17" />
</ETHNICITIES>
<STATES>
<STATE id="3510" name="Osler" pop="3063200" abbr="OSL">
<CITIES>
<CITY id="95040" name="Bereton Park" pop="485000" lat="85" long="70" abbr="BER" />
<CITY id="95041" name="Crenshaw City" pop="309000" lat="35" long="140" abbr="CRN" />
<CITY id="95042" name="Red Tummlyn" pop="655000" lat="25" long="80" abbr="RTM" />
<CITY id="95043" name="Skate River" pop="879000" lat="25" long="30" abbr="SKR" />
<CITY id="95044" name="Fanningana Falls Park" pop="284000" lat="20" long="45" abbr="FFP" />
<CITY id="95045" name="Folgerley Centre Bluff" pop="153000" lat="55" long="110" abbr="FCB" />
<CITY id="95046" name="McKinvenmond Lakes" pop="112000" lat="65" long="35" abbr="MKL" />
<CITY id="95047" name="Pear Falls" pop="94300" lat="55" long="58" abbr="PF" />
<CITY id="95048" name="Powlett Creek Summit" pop="91900" lat="5" long="70" abbr="PCS" />
</CITIES>
</STATE>
<STATE id="3511" name="Pargerveron" pop="3400000" abbr="PAR">
<CITIES>
<CITY id="95060" name="Gregorsoncott" pop="298000" lat="-60" long="-40" abbr="GRG" />
<CITY id="95061" name="Kern" pop="518000" lat="5" long="35" abbr="KRN" />
<CITY id="95062" name="West Somerchester" pop="767000" lat="0" long="-40" abbr="WSC" />
<CITY id="95063" name="Widmertown" pop="1183000" lat="-45" long="40" abbr="WID" />
<CITY id="95064" name="Buck Falls Heights" pop="184000" lat="-25" long="-2" abbr="BFH" />
<CITY id="95065" name="Grand LaValleyford Prairie" pop="162000" lat="-40" long="-65" abbr="GLF" />
<CITY id="95066" name="Hot Bear Bend" pop="133000" lat="-20" long="50" abbr="HBB" />
<CITY id="95067" name="Leadland" pop="84000" lat="-40" long="110" abbr="LDL" />
<CITY id="95068" name="McMurrichswell Cove" pop="71000" lat="-80" long="90" abbr="MMC" />
</CITIES>
</STATE>
</STATES>
</NATION>
</NATIONS>
</CONTINENT>
</CONTINENTS>
<REGIONS>
<REGION id="1" name="USA: East Coast">
<REGION_STATES>
<REGION_STATE id="118" />
<REGION_STATE id="122" />
<REGION_STATE id="125" />
(((snip)))
<REGION_NATION id="251" />
<REGION_NATION id="252" />
</REGION_NATIONS>
</REGION>
</REGIONS>
</WORLD>
I left the region section untouched; as it is unimplemented I doubt it will affect this fictional league and I was unsure if I could simply delete it.
If anyone spots mistakes/problems let me know but it seems to work fine in-game.