FRANKFURT—The European Central Bank said on Monday it plans to redesign the euro’s banknotes over the next three years to make them “more relatable” to Europeans of all ages and backgrounds. ECB focus groups will now gather suggestions about the overarching theme of the new banknotes from the public in the 19 countries that share the euro, the ECB said. The new design will replace the windows, doorways, and bridges of different architectural styles that have adorned the euro’s banknotes since their launch in 2002. “After 20 years, it’s time to review the look of our banknotes to make them more relatable to Europeans of all ages and backgrounds,” ECB President Christine Lagarde said. The original design was meant to signify unity and openness while being abstract enough not to be traced back to any individual member country. But many other Treasuries and central banks have since sought to put …