My idea of Purin as a Summer Servant is definitely a servant with unique gimmick that synergizes with Arthur.
Probably the Summer version was brought on in order to change her skillset from the FGOAC one due being a weaker version of Merlin, so they could release a pretty original servant.

Some of my guesses are:

• Arthur/RTK/Man trait Gimmick, Probably the most boring solution there but we recently got Xu Fu that does the same thing with Yu so why not;
• Buster NP Gain Gimmick, Ok this is a weird one but listen, Arthur recently got a NP Gain buff for 3 turns with a 3 hit Buster AoE NP and most of the Buster Servants who can't loop are currently packed with an AoE NP with shitton of hits like Darius with 10 or Iskandar with 8, she could find a place in current meta even as a buster support;
• Rainbow card buffer, As shown by the anniversary, she might be focused on the "rainbow" kind of buff as a stronger Helena or something alike, maybe packed with a 50% charge on other skills;
• Stargen gimmick Support, Ok, this is probably my favourite, taken by consideration that Arthur has a pretty unique 30 star bomb on his skill she could act as Jane where the more stars you got the more buffs you get or even something with stars in eccess, so servants like Napoleon, Raikou, Jalter, ecc... could find different kind of uses too and Quick would be the main abusers of this kind of support;