In my opinion, the strongest things you can do in Mono Red are: "Glass cannon" decks that either quickly pop off and win before opponents can stabilize, like Storm, Krenko, Purphoros, Slicer Crushing nonbasic lands, aka defeating a large chunk of players with a single Blood Moon The problem with these two strengths is that they aren't always the right pick for casual tables
Essentially it's a more niche Altar of the Brood but you can also mill yourself to win with Thassa's Oracle or whatever
Gwenom, Remorseless can look at the top card of your library at any time, and you may play those cards from the top of your library
Evolution Sage gives you proliferation whenever you play lands
Marcy: I like the joke of a loyal dog that buffs humans, but this feels like a card that belongs in a Human deck, except that it isn't a Human, and also most Human decks are monowhite