I had originally a longer idea but i will drastacly change it and make it shorter because of the response i got.

A group of children saved the digital world with the help of the royal knights or at least some of them, after that both worlds are separated. After some years equestria shows up and the convertion starts.

The war is split between 3 faction: only pony faction, only human faction(THLF) and those who wich peace and coexistance made of oth humans and ponies.

During an attack the children who growed up and survived thus far(because the plot says so) and are part of the 3rd group gain the power from thei friends to transform akin to spirit evolution into the royal knights

5049473 I've heard a lot of people get gungho over crossovers over the years. Most of them, like you, end up posting all of their ideas to the forum. The thing is, by doing it this way, you run out of steam to tell your story, and yes; it is your story. If you want people to write a story for you, you have to write enough of it to invest the reader into finishing it. Case, point, and interesting story: it was a cliff-hanger episode in Digimon Adventure that got me to write a twenty page alternate ending by hand while waiting for the next episode to come out.

I don't really see how Digimon and Equestria could crossover very well unless you reach outside of the de facto TCB universe and go for something less permanent, because the way you described it has way too much going on at once. My suggestion to you, just to make this work as a world, is: start in the more or less present keeping ponies and digimon hidden from the general populous. This means you'll have to remove the HLF factor, because the only humans who will be involved are “the destined” who you can play off as both chosen by the digital world and Celestia respectfully.

I don't really see where you are going to get your conflict as Digimon is very much a fighting-centric franchise and MLP is the antithesis of fighting. The plot you suggested is pretty far outside of the accepted norm, because it is fighting and not only is it fighting on a mortal level, but it's fighting on god level as Celestia and Yggdrasil are both considered gods of their worlds. One thing that's pretty clear is that if you are going to have gods and mortals in the same story, you can't have the gods doing any direct fighting because mortals don't stand a chance of changing anything at that level. All they do at that point is waste the reader's time.

You could direct the ponification to effect troublesome digimon rather than humans, but this drives the plot to a quick and easy solution which doesn't leave much to drive the plot, unless you focus on the internal struggle of the right of anyone to change another's form, but if you did that the fact that digimon are even present in the story might get buried.

The only way I have ever seen a crossover work is to first ignore the prime focus of either universe and focus on building something that puts the overlapping under-appreciated aspects of both worlds as the primary focus. Professionally, I don't think authors give a lot of thought to crossovers, but my opinion on the matter is that they are insanely hard to pull off well and take a lot of passion for both, so if you're going to go for it, then think things through and make sure you know how things can go, before you figure out how they will go.