Matilyn/Mattilyn/Matilen nn: [name]Matti[/name]
Instead of [name]Madeline[/name],[name]Madison[/name]
It’s definitely different…normally I don’t like names that aren’t run-of-the-mill but this one’s growing on me 
No offense intended here at all, but I really dislike it. Mattilen sounds the exact same as [name]Madeleine[/name] and it looks illiterate to me. It’s sentencing a kid to a lifetime of “Mattilyn, that’s M-A-T-T-I-L-Y-N,” repeated four times to everyone they meet. If you want [name]Matti[/name] over [name]Maddi[/name] (they sound the same in my area of the country, but if you prefer the look) I’d go with [name]Mattea[/name] or [name]Matilda[/name]. If you want a unique name, pick something unique, not something ordinary spelled differently. [name]Matti[/name] or [name]Maddi[/name], she’ll still be one of three in the class.
I’m sorry if that sounds harsh, but I wanted to give you an honest opinion.
I don’t care for alternate spellings when they do nothing to change the pronunciation of the name. Mattilyn and [name]Madeline[/name] would be pronounced exactly the same in my region so it’s just confusing. Honestly it looks like Dad was [name]Matt[/name] and wanted a boy but got a girl instead. 
I agree–creating a nu-spelling isn’t altering the name. Some names, like [name]Catherine[/name]/[name]Katherine[/name] or [name]Elisabeth[/name]/[name]Elizabeth[/name], are given variant spellings becuase they are classic enough that the spellings are both widely accepted and have been for perhaps centuries (or millenia, in the case of [name]Elisabeth[/name]). Changing /d/ to /t/ and /i/ to /y/ (or vice versa, and etc.) and repeating random consonants multiple times aren’t things that enhance a name or improve upon it.
I don’t like it at all.
I’d rather [name]Matilda[/name] or [name]Madeline[/name] or [name]Mattea[/name] over Mattilyn. I can guarantee that people will secretly roll their eyes when they hear it.
Let me just say that I work birthday parties at a store for little kids and these spellings are becoming increasingly common in my area. I have to write the names of all the kids attending on what they make at my store so I have to learn a lot of spellings. And every single time I have a [name]Madeleine[/name]/[name]Madeline[/name]/[name]Madelyn[/name] I ask, “Is it spelled M-A-D-E-L-Y-N?” And then the mom always gets this super-offended look and is like “No, it’s M-A-T-T-I-L-Y-N. Or just put [name]Mattie[/name].” And then I have to smile politely and refrain from rolling my eyes.
Personally, I think if you’re going to name your kid something pron. Mad-uh-lin, please spell it [name]Madelyn[/name]. The French spelling, [name]Madeleine[/name], is actually pronounced Mad-[name]LEN[/name] (there’s a tiny ‘ah’ sound between those but not even really when you say it fast), and if I see one more [name]Madeline[/name] mispronounced as [name]Madelyn[/name] I might bang my head against a wall. Mattilyn I always want to pronounce [name]Mat[/name]-tee-lin.
Any of the many spellings of [name]Madelyn[/name], however, would be totally fine.