\(
\require{cancel}
\require{color}
\newcommand{\p}[1]{\left(#1\right)}
\DeclareMathOperator*{\argmin}{arg\,min}
\DeclareMathOperator*{\argmax}{arg\,max}
\newcommand{\given}{\,\middle|\,}
\renewcommand{\Pr}[1]{\mathop{{}\mathbb{P}\mathopen{}\left(#1\right)\mathclose{}}}
\)
The Problem with the Problem
Obviously, the answer for the base case ($h = t = 0$) is $p^{h'}$, so we
need to know something about $p$!
Either we must change the question to something more manageable, or just assume a “prior” $\textcolor{orange}{\Pr{p}}$ for $p$.
Our assumption about the prior is what makes Bayesian approaches
subjective.
Frequentist (point estimate) approach: Maximum Likelihood (ML) solution
Use the
single most likely
value of $p$ to estimate the answer:
\begin{align*}
\Pr{h' \given h, t}
&\textcolor{red}{\approx} \p{\textcolor{green}{\argmax_{p}\,\Pr{h, t \given p}}}^{h'}
= \p{\argmax_{p}\,p^h \p{1 - p}^t}^{h'}
= \p{\frac{h}{h + t}}^{h'}
\end{align*}
A couple of examples:
\begin{align*}
\Pr{h' = \phantom{0}2 \given h = 10, t = 4} &\textcolor{red}{\approx} \p{10/\p{10+4}}^{2\phantom{0}} \approx \textcolor{red}{\mathbf{51.0}\%} \\
\Pr{h' = 20 \given h = 20, t = 1} &\textcolor{red}{\approx} \p{20/\p{20+1}}^{20} \approx \textcolor{red}{\mathbf{37.7}\%}
\end{align*}
Pros: easy to compute; does not require any assumptions regarding $p$.
Cons: does not answer the original question; approximation error is unknown.
Use the
single value of $p$ that maximizes the likelihood
of the tosses to estimate the answer:
\begin{align*}
\Pr{h' \given h, t}
\textcolor{red}{\approx} \p{\argmax_{p}\,\Pr{p \given h, t}}^{h'}
&= \p{\textcolor{green}{\argmax_{p}} \textcolor{green}{\Pr{h, t \given p}} \textcolor{orange}{\Pr{p}} \div \cancelto{(\text{unaffected by }p)}{\Pr{h, t}}}^{h'} \\
\end{align*}
Pros: easy to compute; can account for a non-uniform prior $\textcolor{orange}{\Pr{p}}$
Cons: does not answer the original question; approximation error is unknown; requires $\textcolor{orange}{\Pr{p}}$.
Notice that if all values of $p$ are equally likely ($\textcolor{orange}{\Pr{p}} = 1$), then this is the same as the ML solution.
Fully Bayesian (exact) approach: “Bayesian” solution
If we already know $\textcolor{orange}{\Pr{p}}$, then we can just find the exact solution directly.
There is no need to estimate a
single value of $p$ when we can consider
all possible values:
\begin{align*}
\Pr{h' \given h, t}
&= \int_{0}^{1} \Pr{h' \given p, \cancel{h, t}} \times \,\Pr{p \given h, t}\,dp && \text{(marginalize)} \\
&= \int_{0}^{1} p^{h'} \times \,\frac{\Pr{h, t \given p} \textcolor{orange}{\Pr{p}}}{\Pr{h, t}}\,dp && \text{(Bayes's rule)} \\
&= \frac{\int_{0}^{1} p^{h'} \Pr{h, t \given p} \textcolor{orange}{\Pr{p}}\,dp}{\Pr{h, t}} \\
&= \frac{\int_{0}^{1} p^{h'} \Pr{h, t \given p} \textcolor{orange}{\Pr{p}}\,dp}{\int_{0}^{1}\hspace{1.35em}\Pr{h, t \given p} \textcolor{orange}{\Pr{p}}\,dp} && \text{(}\href{https://en.wikipedia.org/wiki/Law_of_total_probability}{\text{law of total probability}}\text{)} \\
&= \frac{\int_{0}^{1} p^{h'} \cancel{\frac{\p{h + t}!}{h!\,t!}} p^h \p{1 - p}^t \textcolor{orange}{\Pr{p}}\,dp}{\int_{0}^{1}\hspace{1.35em}\cancel{\frac{\p{h + t}!}{h!\,t!}} p^h \p{1 - p}^t \textcolor{orange}{\Pr{p}}\,dp} \\
\therefore\ \Pr{h' \given h, t}
&= \frac{\p{h + h'}!\,\p{h + t + 1}!}{h!\,\p{h + t + 1 + h'}!} \div \frac{\cancel{\p{h + 0}!}\,\cancel{\p{h + t + 1}!}}{\cancel{h!}\,\cancel{\p{h + t + 1 + 0}!}} && \p{\text{if } \textcolor{orange}{\Pr{p}} = 1}
\end{align*}
In contrast with previous examples, we find that:
\begin{align*}
\Pr{h' = \phantom{0}2 \given h = 10, t = 4} &\textcolor{blue}{=} 33/68 \approx \textcolor{blue}{\mathbf{48.5}\%} < \textcolor{red}{\mathbf{51.0}\%} \\
\Pr{h' = 20 \given h = 20, t = 1} &\textcolor{blue}{=} 11/41 \approx \textcolor{blue}{\mathbf{26.8}\%} < \textcolor{red}{\mathbf{37.7}\%}
\end{align*}