There will be big gains in Eastern Europe, Italy, Austria, Sweden and Germany for eurosceptics.
So what are we talking? 20 seats? 30? 40? 50? They need at least 30 to just keep up their current share of the seats.
Now, if we base our assumption on national polls (which we shouldn't, but whatever)
Germany: +8
Italy: +13 (a bit more if two eurosceptic parties manage to get over the threshold)
Austria: +2 (Probably only +1 but let's give them the benefit of the doubt)
Sweden: +2
Poland: +5 (Probably only +4 but again I gave them the benefit of the doubt) and if you count Kukiz 15 as eurosceptics, +2
So that's actually exactly 30. Now, I'm gonna say FvD gains four seats, and various other eurosceptic parties somehow manage to gain another 10, so that's a seat gain of 44. I've been fairly gracious, so I think that's a pretty accurate number. At the same time, they lose 19 UKIP, 18 Conservatives and one bloke from DUP, so that's a loss of 38. Net gain...six. Revolution.
Looking at groupings is misleading imo since a lot tag along with EPP/PES anyway.
Yet for finances, support, committee assignments and some other stuff it's vital. Political Groups are not bound of faction discipline (especially eurosceptic ones who are pretty inactive in the European Parliament anyway), but they're still fairly important.