The oath as the law both leave a lot of leeway (spelling?) on how to deal with rivals. Wizard's war, politics, certamen or just plain being a pain in the behind by contesting vis-sources and the like.
As the number of vis-sources lowers, the agression of competing magi will raise.
Of course, there is always the option of pitting the players against one who has fallen from the embrace of the order. A renegade or a diabolist is a strong oponent that can be quite interesting if not made into a stereotype.
Perhaps the Diabolist that sold his/her soul not for power, but for love or to be able to do what they consider necesary?
A renegade Bonisagus who has sworn to share his findings... and has accidentially found a way to make the parma as effective as a shield made out of balsa-wood against a warhammer?
What do you want? Enemies in general, or enemies on the physical "I'm gonna beat the crap out of your ass with a flagpole!" mood?
The former can take a multiplicity of forms. Political enemies and enemies that spread bad reputations for you (can be the same, but we have a specially nasty itinerant enemy merchant) so your socuial interactions are affwected can be there as well. We specially like a similar conveant to ours. We symathise with them since we are pretty much in the same situation, but they are also rivals over resources. Sometimes we collaborate to defend outr shares territory in front of the depredations of 3rd parties, but we also want the same resources, so it is low level warfare most of the time.
The later can be modelled using mythic companions or (even better) using adhoc enemies. For example, create a hedge magus that has control over the animals and oplants of his forest. if you want him to be a real match, make him able to use his powers while you cannot detect him in his forest, and give him +15 magic resistance becasue he carries several magic trinkets with him. Done The system is flexible enough to allow you tio do just that without breaking the rules: you adapt them to the results you weant. Remember that rules are tools, not fixed unchangeable realities. If you need to modify some part of them or handwave them to create a really cool story your players are gonna LOVE, do it! If they cannot understyand WHY something is happening according to HERMETIC rules, the better: we are talking about a weird HEDGE magician: he can do stuff they cannot while he is pants at doing most of the stuff the PCs find easy as hell. Creates a sort of shifting balance as they circle around each other to see where they can strike at the other.
Being hermetic magi doesn't mean you're buddies with every other magi, you can very well hate another hermetic magus, just as you can hate one of your co-workers (and hermetic magi aren't even that close)
Political ennemies exists and can make your life miserable, and any enmity could very well degenerate into wizard's war
This is even the official policy of hedge magics. Less powerfull, but sometimes ignoring hermetic restrictions
Any creature with a high soak can be a problem...A low Might will tend to discourage spont spells to disable it. If its soak is high enough, PoF and such won't hurt it....So if you have standard combat magi, it can be a hassle. Of course if the Magi have more interesting combat spells, they can fight back...
I have to recommend one of my favourite bad guys that i have introduced into my saga.
My saga's magi were happily strolling through a faerie forest at night when they met a group of pasty, feral looking humans. After a brief bit of banter the clouds parted, the full moon shone down and the 7 humans all turned into nasty wolfman type monsters.
The ensuing battle was a total rout for the magi. The wolfmen had a MM of 25 and had quite good combat stats (although not at all insanely so). This combined with the outnumbering (3 magi, 7 wolfmen) led to a series of light wounds to start with, as the magi saw their first spells bounce off the monsters MR. As rounds progressed the progressive wound penalties made the magi fare worse and worse in combat and the wounds get nastier and nastier. Eventually, the two magi that could teleported away and the third flew off. All told only one wolfman had died.
My players aren't used to being defeated and to be trounced by relatively weak bad guys was a bit of a wake up call.
So good baddies to throw into combat, MR25 wolfmen.
As for using hermetic magi as baddies. My saga features some not-so friendly infernalist Diedne with reasonable MR. Perfect fodder for baddies. Combined with a subversive conspiracy amongst some members of the order and paranoia and covert violence have been the rule of the day.
I am actually in the process of making a Hedge wizard with Parma.
This Hedge Wizard found a Old Book on Parma, but couldnâ€™t read it and didnâ€™t know what it was, until my players came by, with a note in Latin, witch they helped her translate. With this note and some other stuff, she learned enough Latin to read the book.
Maybe this Hedge Wizard, will sell this information to other Hedge Wizards, and the Parma I Lose in myth Europe.
One of my players has a lot in Mentem, and can turn most attack to his advantages, and then a big soak is not a solution.
But on the other hand he always has his shield grogs with him, and they can easily handle at Creature with a high soak.
They've been very crafty with the use of apprentices, and some of the changes between editions (we started in 4th and transferred to fifth) have made them more powerful than they would other wise be, but none of them are ridiculous, even if they do have a way of working the rules to their advantage
Arts scores, we are looking at nothing beyond 15 as their very highest arts, usually far less. Magic theory, they have all really worked at and is at least 10 in the case of all three magi. Penetration and finesse around the 4/5 range. Only one familiar and he is rather mild. They have seen lots of action however and are quite good at kicking rears when rears need kicking. And often when they don't.
Most of them have also invested in protective magics and wards that protect them from wood/metal/etc.
Due to the differences in warping between 4th ed and 5th ed, we've got a variety of effects that don't cause warping. i.e. stuff they spent a while working on in 4th ed and is rendered much less useful because of warping in 5th ed, so we fudged a bit and those earlier effects don't cause warping but anything invented now does. There isn't much in this.
Great theoreticians like them will probably attract jealous rivals. Especially if they have written books. If they haven't, well, you can always have a disaster hit their libraries, other covenants want to trade book for book -- they don't have the art scores to write great books -- and you'll have them.
Then you indeed need political ennemies. I wouldn't like to bear the disfavour of a Primus...
That's another point of interest for the great theoreticians of the Order. They'd probably be interested to hear about that. If the players refuse ... instant ennemies.