IF the UK is still a member by May 23rd, five different aspects will be affected:
As long as the UK is an EU member, they have the right to be represented in the EP. From an EU-27 perspective, this might be treated as a UK problem, but these these British are still EU citizens and would have good a good case in court.
As long as the UK is an EU member, they have the right to vote (& stand!) in the EP elections. Should legally not be a problem, but might slightly affect the result in areas with many British citizens. The least problematic bit.
This is most problematic - if there are no EP elections in the UK, but the UK is still a member, these @the3million can neither vote in the UK nor their EU countries. Thus #Brexit would rob 1 million+ EU citizens of their vote. Unacceptable.
If the UK holds #EP2019 elections and is still in the EU by July 2019, its MPs will affect the makeup of EP political groups, the majority in the EP and the next Commission. 73 MEPs with an uncertain future will make finding majorities much harder.
If the UK would fail to organise EP elections and the EP thus had to convence without British MEPs, all decisions this 'EP27' would take could then be questioned in court. This would cast a huge shadow over EU decisions in critical times.
a) a short technical extension until end of April, if Withdrawal Agreement already passed. Extending into 22nd of May would lead to #Brexit overshadowing #EP2019.
b) an extension until the end of 2019 at the least, to secure citizens rights, the EU institutional transition and give the UK time to find a different political path either by general elections or a 2nd referendum or both. /ends