Logia users are hideously powerful both offensively and defensively and are only really countered by something very specific in the powers roulette, which they can often counter themselves such as Crocodile with his dehydrating abilities, and then there's seastone and haki, both of which are rare. The fruits themselves are also incredibly rare to boot. There's a good narrative reason why Luffy's crew has no logia users to this day and the majority of the Logia users are enemies or not directly allied with the Straw Hats. If the goal is starting from the bottom and heading up then I feel starting with a logia should be banned, same with mythical zoan fruits which are supposedly even rarer and oftentimes just as powerful. You could introduce them as rewards for good players with no fruit powers later on, if they want them. Well, that's just my opinion.