Thanks for your suggestions!

(yeah I looked into simple synonym searches as well, I especially like the "Phillistine" for Barbarian class name )

I suggest you try and think about your names in a different way. What do you want your names to communicate?

Class names communicate in two ways: they tell you what the class is about, and they communicate the flavour of the game. Bog standard names are better for the former. If you tell me you have four classes called Fighter, Cleric, Wizard, and Rogue then - boom - I know exactly what those four classes are. You've communicate a lot with a little, and that's really not a bad thing. Of course, you also need to make sure that the names you've chosen actually are communicating correctly, for example if your Cleric is not a midline fighter with healing magic but a knowledge-based character class with no magic or combat capability then it wasn't a great choice of name.

If, on the other hand, you have the same four classes but call them Samurai, Chiryo-shi, Wu Jen, and NInja then you've communicated less but set your game up for some kind of Japanese themed adventures - at least to those of us who don't know much about Japan. Call them Iron Heart, Exalimo, Quentarch, and Imnolitor are, well, you've communicated nothing and must teach the concepts to the player. This can help you make your game, and world, seem alien and unfamiliar but it comes at a price and, honestly, if your not making your game mechanics sufficiently distinct to go with that it'll seem like you doing it for the sake of it.

So, what do you want your class names to communicate about your game, and why?

Good points Irusan. 

I start to think my names are pretty good as they are. I might make some minor adjustments, but as my game borrows from both Battle Brothers and Darkest dungeon this might work fine for now.

The theme is old school (almost boardgame-like) dungeon crawler with both high-fantasy (potions and magic) and a grittier medieval, survival theme to it. The classes cover rpg tropes such as dps and tanks but strict healers are not present (as healing is a part of the survival aspects of gameplay).

The third leg of the classes (carried by mainly the cleric, ranger and priest classes) is support rather than healing (protection, buffs and crowd control).

I might change "Mercenary" to "Man-at-arms" even though he is not mounted. Sounds pretty cool for a soldier-like weapon expert and feels more heroic.

Going further than Irusan's good advice about what class names mean for players, think about what they mean in the game's fiction. Sticking to your OP list:

Strong frontliners:

• Templar: "knight templars" are supposed to be more religious than generic knights, and the name implies a temple: what temple? And more generally, what religion? And why should they be distinguished "strong frontliners" if religious activity steals time from training? At best they should match plain knights on the battlefield (with other types of ability to compensate).     
• Barbarian : it implies immigration (or in extreme cases, like ancient Greeks in the Mediterranean and Europeans in Greenland and America, emigration) and strong ethnic/cultural prejudice (we are civilized, they are barbarians). It doesn't correlate with being a "strong frontliner", or with any specific character class, unless you embrace very specific and fairly bad D&D stereotypes.  
• Cleric (healer and knight sort of. Paladin style): again, specific and dangerously vague D&D stereotypes. A cleric is, generically, a member of the clergy, and the most dangerous realistic clerics, in the common sense of the word, should have some sort of PDW and little training to use it (making them some of the worst "strong frontliners"),
  You are probably giving a misleading name to something else: a knight templar as discussed above (or its differently named counterpart from another cult), a warrior of some kind who changed career, a blessed smiter of accursed undead but not much of a priest, and so on. Again, what religions are we talking about? They aren't likely to share the same attitude towards mystical warriors.
• Black knight: implies a tradition of evil black knights, which is a fairly strange thing to fit in a game setting; think of all the accompanying baggage, from dangerous damsels to enchanted castles, they have in Arthurian poems.

Medium range, agility based:

• Mercenary (soldier type, lots of weapon flexibility): in what wars? for whom? From what background? A period of frequent small wars is almost required for the existence of mercenaries. Being a mercenary doesn't correlate with any particular battlefield specialty. Whole armies, except for the highest officers and some special cases, are likely to consist entirely of various types of mercenaries.
• Assassin: why would an assassin participate in a dungeon crawl? People to kill in a big city should be an abundant and renewable resource. Who becomes an assassin? How do they train? How secretive are they, on a scale from the prestigious guild that serves as an elite school in Terry Pratchett's Discworld novels to telepathic contact with customers in dreams?
• Ranger (outdoorsman type of class): skill at hunting and willingness to shoot people are two different things. Why are these rangers violent enough to go on a dungeon crawl?  
• Archer (specialist in bows and crossbows): after knights, the class that requires the most military training. What army are they from? Have they left it?   

Squishy magic users:

• Sorcerer (classic damage class): a very generic word. What are their sources of power? 
• Priest (support and healing): like a sorcerer, but with divine powers. If they go around supporting and healing, not likely to be actual priests. Exactly the same idea as a "cleric", and only marginally more squishy.   
• Warlock (dark magic): a fairly pejorative word, because "dark magic" is bad! How are they more evil than generic sorcerers? Why are they tolerated?   

While not wrong Lorenzo, a strict etymological dissection of the words might not be so useful. As evident from this thread alone, a word like "cleric" is in rpg-games much more linked with magic and fighting than with religious clergy work.

But you raise a better point with black knights, assassins and warlocks. I must decide how "evil" they actually are in my lore and if they can go together in a group of "good heroes". Why did they join, and why are they accepted by the more traditional hero classes?

The overall "goal" of the heroes in the game are not really set so far either. It should mechanically be along the lines of ridding the lands from a great evil. The group travels further east and will eventually be faced with the "final dungeon" where the great evil will be defeated. Good old-school fun!  I will start another thread on that subject.

There's an easy way out of vague and misleading character classes: you can simply list a fixed set of characters and let the player pick some of them as a character party. This way you can explain character backgrounds and information about the setting and also include exotic and unique characters. For example, not "archer" but either or both of farmer Jimmy now 28, drafted two years ago, veteran of the 4th archers regiment on the winning side of the last succession war, and recently returned to his farm and his family with an itch to be a hero, and sgt. Mary, one of the most promising and intimidating new members of the capital's police forces, with high-tech inclinations that made her an expert of crossbows, firearms and forensics.

That would work better if you had a single hero and not a lot of other stuff to handle. In my game you have 6 heroes and they die and are replaced every now and then. So hero classes needs to be destinct and quickly communicated to the player.

How about making composite classes? i always say that consumables are under developed so its a way to give versatility without making many changes

Alchemist priest (apothecary?): spend healing potions for increased recovery magic

Alchemist assassin(poison master?): spend poison vials to increase damage 

Man-at-arms: automatically equips the armor or weapon that has resistance or advantage against an enemy, basically sacrifices inventory space for versatility

Mage archer: spend arrows to increase spell damage

Defender: spend elemental consumables to increase elemental resistance

Attuned swordsman: can store a spell cast on him by an ally and unleash it with the next attack

you can also use typical production or non combat classes

Blacksmith: bonuses to equipped gear 

Villager: can forage items from the scenario, if the items are worth it this can make a class very valuable even if its not a fighter

Scholar: increases the team's likelyhood of critical hits, only fights by using consumables, maybe can use two every time?

Ambassador: can only be used in events related to their country, units from that faction decrease their critical rate, cannot attack but cannot be attacked, a meat shield

Well this looks interesting to me not sure for others