Rares are only worth keeping in one of two scenarios:

1) You're a completionist and like to just amass a catalog of all of the unique champions like me

2) The rare is a very specific champion that meets a very specific requirement for you, and generally only if you have nothing better

Examples of 2 include Warmaiden, but, as you indicated, you have a better version. Other examples include Frozen Banshee for CB, Metalshaper for scarab in DT, Reliquary Tender for venom in DT, etc... Basically - rule of thumb should be, if you have an epic or a lego that does the job better, use it.

Specifically for FW, I can't comment from experience, since I'm not really trying to max that yet - possibly there are some rares that shine there, but I can't answer you on that one.

From your epics, again bearing in mind a lot depends on who else you have, the ones which stand out would be Doompriest and Steelskull. Whether or not you need them will depend on who else you have. Unfortunately both are pretty much eclipsed by Scyl, so if you already have Scyl I wouldn't really bother with either. The problem with both DP and Steelskull is that they do pretty much nothing else except provide healing, and usually you want your champs to do more than just that. Scyl provides a huge amount of extra value, from the stun on A2, the TM from A1, and the revive from A3.

I've got very little of anything, nowhere near enough to build specialized teams for different purposes yet. I'm just on day 39, so I'm a long, long way from Scyl. The only epics I got (prior) are Shaman, Serris (who I'm very happy I got), Coldheart (who needs much better gear to be effective) and High Khatun (which I'm not sure what to use for yet, as I don't have enough damage output for a speed team in the arena). I'm mostly using a combination of those (High Khatun not so much) and Athel, Spirit Host and Apothecary that I got just a few days ago. I also got Coffin Smasher, mostly for his multiple hits on basic attack to deal with the Fire Knight shield. I was previously using Courtier and Warpriest, but they are pretty much retired now.

So for now, I'm mostly just trying to build up a decent variety so I can advance in the dungeons and get better gear. If they are not all useful long-term, that's fine, as long as I can inch my way forward a bit at least.

If you haven't already maxed Khatun, I would do her next. She's amazingly useful - honestly, the only other champ I've found that can truly take her place is Arbiter.

Maxing DP or Steelskull isn't necessarily a bad idea, just be aware that they'll eventually become useless.

Please  dont  invest  any  time  in  Mistress  Of  Hymns,  or  at  least  dont  put  any  books  in  to  her.  You  will  replace  her  quite  quickly.

Khatun is 5*, my issue seems to be to gear her properly. I'm also hesitant to give up a slot for her, at least not without some more reliable damage mitigation or revives. Apothecary provides both speed and healing, but unfortunately not enough healing to keep everyone alive and little damage, otherwise I would toss Shaman out. Athel is my only real damage and Serris provides too much utility to pass up. I use Coldheart in some places, Spirit Host in others, so those would be the ones I could swap out.

Jareg is solid up to 2 key nm on a poison team. 

As is frozen banshee

Don't underestimate the value of speed. Khatun provides not only a team speed buff, but also a team speed buff aura, AND an enemy speed debuff on both her A1 and A3.

She's incredibly useful in arena, but also very useful in many other places as well. I would 100% max her.

Alright, thanks. I'll keep it in mind. I think I'll focus on Doompriest for now, at least get her to 5*, she seems like she'd fit well in a dragon team with her healing and debuff removal to counter the dragon's AoE debuffing, and I've already put a lot of resources into Apothecary (perhaps a bad decision in retrospect), so I got some of Khatuns kit with him.

I've pretty much given up on the arena since it takes way too much for me to find a fight I can win, but maybe once I get better gear and can get Khatun up speed-wise, I can give it another try.

Doompriest, Klodd Beastfeeder, Steelskull are all excellent support champs. 

Frozen Banshee is top teir for Clan Boss.

Jareg is really good for Clan Boss too.

Ogryn Jailer and Purgator can hit SUPER hard when built correctly. 

You should save all your heroes.

You will need a lot of them for Faction Wars.

Even heroes you don't think you need will be very useful.

For Example:

Warmaiden does AOE Def Down.

You don't want to use her because you own Madam Serris.

However, You can't use Madam Serris in the Barbarian Faction.

Madam Serris is a Dark Elf.

You will have to level up Warmaiden for Barbarian Faction One Day.

Otherwise, your Barbarian team will be missing AOE Def Down.

Purgator only hits hard when you can debuff yourself (Gurptuk Moss-Beard). Otherwise he is just an AoE-attacker with rather low damagemultiplier. 

Yes, as I said, I'll keep them around for the crypts. It's more a question of whether they are useful for anything else and worth investing resources in at the moment. Otherwise, I'll keep them in storage, maybe give them some simple gear just so I can pass the first few levels of the crypts at least.

Madam Serris & Warmaiden are equally good in similar locations.

- Arena

- Dragon

- Faction Wars

- Minotaur

- Most of the Potion Keeps

You probably don't need to use Warmaiden right away because you own Madam Serris.

However, I would slowly invest in Warmaiden over time.

I would slowly level her up.

I would slowly ascend her up.

I would use Dupe Copies of Warmaiden as books.

Keep in mind, There are 13 Factions in Faction Wars.

You own Madam Serris + Warmaiden.

Those are 2 AOE Def Down heroes for 2 Factions.

You still need 11 more AOE Def Down heroes for the remaining Factions. lol

Most AOE Def Down heroes will require you to skill them up with books.

This is why Warmaiden is considered so amazing.

Warmaiden can be booked up with out using books.

You can farm Copies of Warmaiden in Campaign and use them as books.

It can save you a lot of resources.

Instead of using Rare Books on Warmaiden, you can use them on another faction.

Again, you are not far enough in the game to worry about Faction Wars.

I just wanted to bring this to your attention so that you can sort of think about some of these issues which you will face in the future.

Your main focus should be completely all the Arbiter Missions to get Arbiter.

After you acquire Arbiter, you move on to the Lydia Mission.

The Lydia mission is to beat all of the Faction War Crypts to get Lydia.

After you acquire Lydia, you move on to the Doom Tower Mission.

The Doom Tower missions have secret rooms which allow you to get champions.

Right, I'm not too worried about the crypts yet, but I do try to use up all the keys to complete the advanced quests and get some glyphs. I don't really intend to waste much resources on going very far there as of yet, but I've managed to get past 2 of the bosses at least. I'm using the sparring pit to slowly level up the champs I'm not using, and putting some basic gear on some just to make them able to complete at least the first level of crypts.

Right now my main concern is dungeons and improving my gear. The potion dungeons are no issue, I can auto most of them on 15. Minotaur is iffy on 15, but it can be done. Affinity makes 14 bad but doable for me, but 13 is consistent. Ice golem and dragon I can do 14 on. Fire knight only 11, maybe 12 if I do it manually (the AI is really dumb on auto...). The spider only 10, and not on auto.

Getting Arbiter is not going to happen for a long time though. The arena is a hard block on progress there with the ridiculous matchmaking it uses.