Hi I am new to the world of board games but i really would love to start designing

first thought on your concept is you need more options of missions there, that one missions will get really old really fast and I think miniatures games are pretty high budget for something without much content and replay value

*but* you don't have to work that out straight away, you can come up with new missions later

I think it's ok to work out your "fancy characters" in loose terms, like "I want this character to be able to move really fast" or "I want this character to be able to dissapear for a round and reappear somewhere else" or whatever, but you do need to work out the core mechanics that apply to everyone before going into the specifics of how each character works differently. For the purpose of getting the core mechanics worked out you could skip any character-specific abilities entirely.

There are a bunch of different ways of doing miniatures movement.
Just in terms of units of measurement you've got square, hex or region-based movement vs inches or centimetres movement

Then you've got the question of how much can a character do in a turn, they can move but what else can they do? How many different things can they do in a turn?

We can give more pointers if we know a bit more of your mechanics.
Sure character traits are nice to know, like a fast or durable character. But it depends on the mechanics that you use. But also the game where the mechanics work in.

For example, how is the board that you have in mind for this game? Do characters need to walk around corners? Do they need to act from a distance? What happens when they run over the board ignoring the others? Doors, traps or other path blockers?
This is for the fast characters important.

These questions might be legit or nonsense depending on your mechanics.
It could be that your mechanics will be needing new rules or that your rules require a new mechanic.

Now, before you continue.
Get a pen, get paper. (Noooo computer, trust me, take these notes while working/testing on your board game) Start making notes on every thing you think of. Everything you have tested. Your findings are important. And no matter how good or bad the results. Also try other idea's that you had.
You even might sort your notes, depending on the mechanics.

PS. I am a specialist in failing ;)
Don't be scared to fail.