If WotC were willing to risk permanently and dramatically changing all future games of Magic by introducing Companions, then there shouldnt be much of an argument about making single-use, bankable mana tokens a part of most every game going forward
They gain vigilance, indestructible, and haste until end of turn
Cards like Dark Bargain that also provide selection (and exist in a slower format) often overperform, but Blood Pact is awkward
This isnt the first time, and likely wont be the last, that I reference the semi-finals match from #SCGCOL that Brennan DeCandio was able to win almost singlehandedly on the back of Walking Ballista: There is a point in this game in which Brennan has enough mana to beat Hunter while hardly attacking
Fan favorites like Luigi Pikachu and Poncho-Wearing Pikachu are highly valued by collectors
The cost is a bit steep as youll need to keep your Harmonized Trio around and tap two untapped creatures, but I can see it being particularly useful in Izzet spellslinger decks where you need regular small spells