It's really up to the individual state conferences on how they run their own primaries/caucuses - where state laws don't exist regulating them, at least.
Some approaches taken in the run-up to the 2020 election included switching delegate allocation to a 'winner-takes-all' system rather than a proportional system, or even cancelling primary contests or caucuses altogether.
Switching to a 'winner-takes-all' system - where a candidate with over 50% of the popular vote would be assigned all the states' delegates - would obviously benefit the front-runner, especially if applied in states with early primaries, as it would help prevent challengers from attaining sufficient momentum to become a serious challenger. This approach was taken in 2019 by Rhode Island Republicans, and while it wouldn't have had much of an effect in that contest, it could be attempted in more states for 2024. The RNC rules were altered for the 2020 cycle to remove a provision which stipulated a proportional allocation system for delegate assignment if the primary contest was held prior to March 15.
Additionally, in the run-up to the 2020 election, many state Republican parties voted to cancel their presidential primaries and assign all their delegates to President Trump. This is fairly common among both major parties when an incumbent president is running for re-election - Republicans cancelled primaries in 1992 and 2004, while Democrats cancelled primaries in 1996 and 2012. As far as I know, cancelling a presidential primary for a non-incumbent would be unprecedented in the modern primary system, but technically there's nothing stopping state parties from doing so - depending on individual state laws.
In Michigan, for example, state law prevents primaries from being cancelled altogether. However, in 2019, the state Republican party voted to increase the delegate threshold - the percentage of votes needed to be assigned delegates - to 20%, clearly disadvantaging challengers to front-runners.
Finally, another change made in 2019 was to abolish the Republican party's "Standing Committee on Presidential Primary Debate". From the party's 2016 rules:
The Standing Committee on Presidential Primary Debates shall have the
authority to sanction debates on behalf of the Republican National
Committee based on input from presidential campaigns and criteria
which may include but are not limited to considerations of timing,
frequency, format, media outlet, and the best interests of the
Republican Party. Each debate sanctioned by the Standing Committee on
Presidential Primary Debates shall be known as a "Sanctioned Debate."
Any presidential candidate who participates in any debate that is not
a Sanctioned Debate shall not be eligible to participate in any
further Sanctioned Debates.
Buzzfeed reported that RNC committeeman Randy Evans said at the time that the change was "intended to dissuade a primary challenge to the president". Given the public awareness of Trump over any challenger, debates could be a vital chance for a challenger to raise their public profile. If this committee isn't reinstated, candidates will only have the opportunity to participate in unofficial debates organised by third-party organisations.
