Have a Platinum Hero coins

The current chance for a 5* from the epic hero summon is 1.5%. If you want to get the odds to a coin toss between getting a 5* and not, you’d have to use 46 tokens. So 5 - 10 tokens is not nearly enough of a cost for a guaranteed 5*. If you factor in HOTM probability, you’re down to 25 tokens for a coin flip, but it’s not clear from your proposal whether you’re including the HOTM as a potential result, or only the classic 5* heroes.

If you’re proposing only 5-10 EHTs for a new 4* token, then I would only guarantee a 4* hero, with an increased chance at a 5* hero.

Also, this is a potential duplicate of Summon Token Upgrades – Trade/Convert Multiple Daily Summons Tokens into Epic Hero Tokens.