The June champions have already been leaked so I won't be counting those. Whether or not those leaks come out to be truth, we'll have to find out come early June.
July - Hercules and Scarlet Witch (Sigil)/Baron Zemo/Bucky Barnes (Winter Soldier)
I've already made a post about the release date of Hercules which you can find
here. Kabam Miike said a while ago that the champion that will be released alongside the winner of the Summoner's Choice Champion vote was already predetermined. Scarlet Witch (Sigil) made her debut in December 2020 and Falcon has a synergy with a mystery champion who I think is either Baron Zemo or a new winter Soldier. The predetermined champion would have had to be thought out well before the Summoner's Choice Champion vote went live and considering, both Wandavision and Falcon and the Winter Soldier were delayed by a year, those characters fit this bill perfectly.
August - Kate Pryde and Nimrod
House and Powers of X get released - Sunspot and Warlock get added the month after
X of Swords begins - Prof X and Apoc get added during that month
X of Swords ends - Stryfe and Jubilee get added the month after
Hellfire Gala ends - Kate Pryde and Nimrod get added the month after
Historically, we've always had 2 mutant themed months in a year. We had one in January so its a case of when and not if.
The X men are currently in a new era in the comics and during or after every major crossover, we've got new mutant champions champs added to the game. The next major crossover event is the Hellfire Gala which is set to begin in early June and end in very late July. I suspect this spot is meant for Kate Pryde

and this spot is meant for Nimrod

.
September - Anti-Venom and Toxin
As I'm sure you all know, Mr Negative has a synergy with mystery champion and Venom called "Healing Touch". I think this is eluding to Anti-Venom
The trailer for Venom 2: Let there be carnage dropped about a week ago and in the trailer we saw a detective who goes by the name of Mulligan. In the comics Patrick Mulligan was the host of the Toxin symbiote so with the release of Venom 2 in September I reckon Toxin will be released. Something else I'd like to add is that there is this slot in the state of the battlerealm.

Toxin seems to fit in more than say Knull for example mainly due to link with Platinumpool.
October - 1 Kabam original and ???
Gabe, the art director at Kabam stated this.

With the exception of 2020, October has been the month where Kabam add their original champs. With regards to the state of the battlerealm map, it makes perfect sense that Kabam will release 2 OC champs this year with the first being in early June.
As for the other October champ I think its going to be whoever this spot belongs to.

November - 2 Eternals
The release date of the Eternals movie is on November 5th in the US so I suspect Kabam will add two of the Eternals. The Eternals that do get released could be literally anyone. Ikaris is one of the main ones and the one most likely to get added and Sersi being the other one. Black Knight, Thena, Gilgamesh could just as easily be the ones released instead.
The spots on the state of the battlerealm reserved for the Eternals I think are these ones

December - Spiderman 2099/Iron spider
December is the month when Spiderman: No way home gets released in cinemas. To capitalise on this, I predict that Kabam will release either Spiderman 2099 or Iron Spider. Iron Spider was in the motion comic for Stealthy but I think Spiderman 2099 is more likely to be the one released due to him having ties with Punisher 2099 who has ties with Guillotine 2099 and Morningstar. 2021 seems to be the year of Kabam with them building on the universe they've created. Releasing Spiderman 2099 would allow them to do so even more.