I encountered this problem with one of my players. They wanted to play a druid character. Everything was fitting except that the druid didn't use wild shape, the character had kind of a "This form was given to me by Nature. Why would I want to change?" mentality and he was trying to be more of a summoner and someone who would commune with spirits. I thought it was cool and I went with it.
But there was the issue with a key mechanic of the Druid, Wild Shape. The character never used it so we tried to give the druid some kind of a replacement.
I ended up giving him an option to burn his "wild shape" uses to hold a concentration of a single spell…
Turns out that being able to use two spells with concentration makes you really OP…
So I would like to ask if you know of any way how this could be balanced? And if you know of any other valid Wild shape replacements.
I thought that I could use the wild shape uses to keep the concentration up in case the druid fails the con check. I am not sure if that would be balanced either. Another option was to ReSpec to Nature Cleric and give him the druid spell list instead of the cleric one. But that didn't fit the character.
Consider drawing inspiration from the Circle of Spores.
The Circle of Spores, published in the Guildmasters' Guide to Ravnica, gives an alternate use of Wild Shape called Symbiotic Entity:
I don't know if this would be on-theme for your player's character, but you may be able to reflavor or slightly alter it to make it work.