Might make 2.5 billion but I doubt it will beat Avatar.
Lest anyone forget Infinity shattered records opening weekend too and didn't even come close. Infinity even beat Force Awakens for opening weekend domestically and then ended up something like 250 million behind it when all was said and done.
While I think Endgame will do better than Infinity I think Endgame will play similarly in terms of week to week declines and thus won't catch Force Awakens domestically or Avatar internationally.