Tucked away in a leafy part of south London, the historic Oval cricket stadium peeking above nearby houses, some 30 developers at Ustwo Games have been striving to solve one of the defining creative conundrums of modern entertainment: how do you balance the familiarity of a beloved franchise with freshness? The problem is exaggerated with Monument Valley, the acclaimed puzzle series that debuted in 2014. The architectural puzzles of the first entry already felt crystalline; the minimalist…