Nemeses don't have a toughness. Nemesis can attack as if they were creatures, dealing damage equal to their loyalty (would probably actually use a different word). Damage dealt to a Nemesis is removed from their loyalty (again, would probably choose a different name)

So they are like creatures that are dealt damage in the form of -1/-1 counters that can't block?

Feels like thematically this would be a group of creatures.

Technically no, since Nemeses would be losing their counters rather than gaining -1/-1 counters. I see the point you're making though, but they weren't intended to exist in a way with with -1/-1 counters (like, they shouldn't be destroyed by something that can destroy something with -1/-1 counters on it).

Instead, references should refer to their counter type, preferably based on amounts (e.g. Nemeses with two or fewer loyalty (or other name) counters gain deathtouch, Exile target Nemesis with five or more counters, when ETBs, put two counters on each Nemesis you control, or Move all counters from one target Nemesis to another target Nemesis).

I viewed these as purely aggressive antagonistic entities, something that must try and destroy and cause harm, which is why I never thought to have them as something that could block. Nemeses can use a and attack in the same turn. Nemeses are clever enough to alter the state of the world around them to make their situation better.

Um. So I get that you're having this as a mirror to planeswalkers. But... if the caster likes the ability; why would the choose to ever make an attack that might be blocked? Which renders them completely safe except to direct-damage decks. Not impossible to balance around, but annoying.

More difficult to balance around - this is both quite a nice utility creature, and a 7/1 smash-to-the-face. And it seems like that's going to apply to all of them; if their ability is usable more than once, they are also a powerful beatstick. Which means they can only exist up in the realms of high casting cost; which means the abilities are either really strong, or completely irrelevant.

Mechanically, as SI points out, they can be implemented as creatures that gain -1/-1 counters. Is this different enough to warrant a whole new type? Personally, I'd say no. But then, I also think that about planeswalkers.

Planeswalkers are different from other types, though. Planeswalkers can be attacked and can be damaged, and are damaged in a different way than creatures are. However, loyalty abilities can be on any permanent (although only planeswalkers will die due to zero loyalty counters as a state-based action) (and I did have some designs that have loyalty abilities on non-planeswalkers, although such a thing will not be common). Planeswalkers are significant to be their own type it makes sense, I think.

This card is different, though. A different word than "loyalty" would be worthless, I think. It is damaged like a planeswalker and attacks like a creature, which is strange. I still think probably is not necessary to be its own type, and then can avoid to deal with the rules that would be required to handle such a thing, too.

It can be implemented as creatures with -1/-1 counters. Another way would be to be both a creature and a planeswalker, and also indestructible, with abilities that it cannot block and cannot be attacked (although maybe it should be attacked anyways, due to the balance; maybe it should also be allowed to block).

I think, the best way to go with this, is to find a flavor-execution for this and reconsider the parts of the design, questioning how you would do that top-down.

Planeswalkers are mechanically different in that they can be attacked, but also thematically different in what they represent - they represent allies that are more comparable to the player - which is why they come with their own "life total" and own "spells". The new card type is only justified through both of these.

We could have gotten legendary creature cards to represent planeswalkers or we could have gotten structures to represent attackable permanent types that track their "health" in counters. Only in bringing the two together planeswalkers got a jump on those other executions IMO.

Your response to -1/-1 counters is misleading. Could do the inverse and start the creature with +1/+1 counters instead, comparable to the phantom ability. Compare this side-by-side with a creature variant that uses +1/+1 counters, maybe even a special symbol for removing +1/+1 counters to activate an ability only once a turn etc. - which I consider independently to be an interesting design space.

Well, flavour is nemesis... Wait. Why is your nemesis working for you? Or is it your opponents nemesis? In which case why are there more than one? Or a nemesis of the planeswalkers they might or might not play? In which case isn't that just one of the other planeswalkers?

I dunno. I guess the real question for "Is this worth making a new type" isn't "Can this be done with an existing type" (Because of course it can. All lands could have been 0-cost artifacts with a restrictionon playing two a turn. Enchantments are just poly-artifacts. Aura are a special case of equipment, nd sorcery are just instants you can only cast in your main phase. ... both ears and the tail.) - but rather whether there's enough interesting designs to keep it around for future sets. Since you'll have to pay the rules-baggage forever.