Semi-long post, kind of a rant. Don't even know if I'll play again but wanted to share my 2 cents and maybe help if its possible.
TLDR at the end if you are to lazy to read.
The reason why these 8.0 (and prior) servers can't last more than a month (Askara lasted 2, and that's like one of the longest ones in last 3 years) is quite simple, there is nothing to do after 2-3 weeks of active play, so playing the game gets stale, boring and repetitive with no payout. But I believe that it can at least be prolonged from a month to 2 or 3 with right changes, especially during Autumn/Winter .
I played both Askara and Kasteria actively, and by the I was 150 level which took around 3-4 weeks since there were a lot of wars and random pking (so it can be done much faster) I have already done all the major quests (INQ, POI, Anhi, DHQ, etc), I have done literally all the tasks, both exp and cash at least twice (some of them like 5 times) and that was about it. Only thing left to do was to grind levels but with no real goal since there is nothing to play towards... I could have exped Grims/Frosts/Eyelords for lvl, or Venoms/Demons/Terribles etc for money, and by that time I was 150 it already was around 1h or more for one level.
And there is that kind of a break point for every Vocation, for paladin its around 150, mages 130 and knight like 160-70 (don't know to be honest, I was paladin on Askara and mage on Kasteria), where you just hit that level for war with enough speed/cash/hp/mana whatever, so it doesn't even make sense to keep exping like mindless zombie all the time, because it doesn't really matter whether you are 150 or 180 RP (or 130-160 mage) you die just as fast, your damage is pretty much the same and there is nothing new for you to exp toward, no new tasks/eq/hunt places, nothing... So you just log out and war and maybe if you need some cash you go on and exp for few hours 1-2 a week... What is even worse, it was better and more rewarding to keep AFK training your distance so people that were 30-40 levels lower than me (which is like 50 hours more of time invested all things considered) did more damage than me, same applies to mages but to lesser extent, but this isn't really the main issue if there was something to play towards other than WAR people might actually continue.
To sum up I would need to spent around 30-50 hours of active play when you include leveling, deaths, taken spawns, selling loot, war, etc to go from 160-180 level without any real difference and nothing new to play for, if there at least was some better weapon/armor/outfit anything at like 200 or earlier doesn't even matter people might actually continue to grind, but as you could see with your own eyes people would level to a comfortable level and just login in again to fight, I did it to at the end too... (Yea you could do it in half the time or less if you go for extreme PG on mages, but those are outliners).
I don't really know what you can do to prolong, I have some ideas, some might be dumb some might be simple and might actually help...
1. More different tasks and ability to reset one task each weak (for example if you use it on Venoms you can do it again and get the 100%, each week, or you can chose Demons next week, you get the point). Adding different and harder tasks could maybe engage RPG players. Like add a different NPC that hard/unique/outfit tasks, like kill 500 Green Djinns for 100 Green piece of cloth (this way you might engage people to do more outfits) or kill 1000 of each dragons for a Raptor Mount, or some from real tibia like kill 6666 Demons for Full on RPG players for completely new outfit/item whatever.
Add daily tasks or something along those lines? New content? Tasks for bosses? 5000 Dragon lords for a change to kill Demodras? just spit balling...
2. 2 hours of Green stamina. This is kinda needed because lock down/quarantine is over, so most of the players from Askara including me, my friends and most of the community can't play for 5-6 hours each day and are limited to maybe 1-2h so they could use these 2 hours to exp and somewhat keep up with the people that are free to play as much as they want... You can add that stamina regenerates faster in trainers to keep the online numbers up if that's your goal.
3. Rotate the war map arena. That war arena was cool but it got boring after first week, maybe use places from RL map and rotate them every few days or each week so even tho we are playing custom MAP we get a chance to fight at the Edron gates/LB fields etc...
4. Just more end game content all around, it would be cool to have to play towards something. If there was really good crossbow at 180-190 people that are already 150+ would go towards that and wouldn't just stop, or a new sword/spellbook/armor with cool resistance whatever... Or like if there are monsters that are really strong and you need 200+ to start hunting them etc, you get the point.
5. Consistent updates
TLDR: No end game content in 8.0 tibia (and prior version) people get sick and tired of grinding an stop playing all together and only war which gets boring after few days as well and the server dies... There need to be some new end game content and constant updates to keep players engaged...