I recommend playing at 200% everywhere.
1. Hard fate. (XL, XL+U, H+U) A very brutal template (1 player against 2 computers who have 8 cities each and absolutely no guards for objects at the respawn). The template isn't the wealthiest for Red and is only suitable for experienced players; I played as Rampart myself, and throughout the game my army strength was always lower, even though I had a wild tempo. By the middle of week 3 or start of week 4, opponents manage to gather a huge army and push into their unguarded treasure rooms. Therefore, if you don't arrive by week 4, there is a chance the game will drag on or you'll lose entirely unless you hit-and-run, because Haven won't be able to stop 8 settlements in a row.
Of interest, many heroes from Campaigns have been unbanned for gameplay variety. Of course, except for Gelu and the Dragon.

2. Revolution(1vs7). Size from XL to G without caves. I recommend setting it to H without caves. You need to set 1 player and 7 computers (make them all allies). Another difficult template where your quite wealthy respawn (value 125) is surrounded on all sides by opponent-allies, with 3 cities each, and all of them will start breaking in from week 3-5 if they haven't died by then :D. The template is similar to the "Everyone Fights" map from The Restoration of Erathia.
https://cdn1.savepice.ru/uploads/2018/9/1/5c22ffa104f21061e7db216e4efadd19-full.png
3. JebusNoAngels. Jebus Cross without Haven and Conservatories, etc. There are simply many arguments about banning Haven in heroes, etc. No need to ban anything; here is just a template where they won't generate—maximum 1 per map. So play with your own armies.

Unpack the archive into the game's templates folder.